université du québec à rimouski facteurs biotiques et abiotiques

UNIVERSITÉ DU QUÉBEC À RIMOUSKI
FACTEURS BIOTIQUES ET ABIOTIQUES INFLUENÇANT LES COMMUNAUTÉS DE
ZOOPLANCTON DES ÉTANGS SUBARCTIQUES ET LA RÉPARTITION
GÉOGRAPHIQUE DES CLONES DU COMPLEXE DAPHNIA PULEX
MÉMOIRE
PRÉSENTÉ
COMME EXIGENCE PARTIELLE
DE MAITRISE EN GESTION DE LA FAUNE
PAR
CAROLINE JOSE
FÉVRIER 2009
UNIVERSITÉ DU QUÉBEC À RIMOUSKI
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Il
REMERCIEMENTS
Premièrement, un grand merci à France Dufresne pour son e ncadrement et sa confiance.
J'a i grandement apprécié la liberté qu ' e ll e m 'a laissé afin de me ner à bien mon projet de
maîtri se. Merc i également pour ses idées, son temps et sa rapidité dans les cOITections tout a u
lo ng de mon parcours. J' ai é normément appri s de ce proj et; j ' ai notamment pu développer
mo n auto nomi e, ma c uri osi té scientifique, j' ai appri s de mes en eurs e t j ' ai gagné un pe u
d' humilité envers m on espèce modè le, les daphnies. Comm ent ê tre si petit, si transparent et si
compliqué à la fois? ! ... Les daphnies ne méritent pas d' être séc hées e t données en pâture à
des vul gaires poissons rouges d 'aquarium'
Merci à la c haire de rec herch e Bi oNord po ur les deux bo urses q u' e ll e m'a accordé
d urant ma maîtri se. Ce fût à c haque fois un ' boost' de mo ti vati on q ui a sûrement contribué à
mon succès.
M erci au Baromê tre qui m ' a permi s no n se ul ement de ' décroc her ' durant to utes mes
é tudes à Rimo uski mais en plus qui m ' a fa it vivre et a payé mes dites études . Q ue demander
de mi e ux?
Un grand merci à Mi c he l G osselin avec qui j 'ai appri s à travaill er, ri en de moin s. Je lui
dois toute mon expérience professionnelle, pour l'in stant. ..
Merc i à Chritian Nozais et Mill a Rauti o d ' avoir accepté de cOITi ger m on mé moire .
M erci à Juli e Demers-Lamarc he pour son aide précie use lors d u teITain de 2007 à
Kuujjuarapik .
111
Merc i à tous mes ami s et à m a famill e, bl a bla. On s' en reparl e ce soir auto ur d ' une
bière .. .
De ux ans . .. c'es t te ll em ent passé vite ! J' en ve ux enc ore!!!
IV
RÉSUMÉ
Ce proje t de maîtri se visait à approfondir d ' une part, les conn aissances sur le
foncti onnement des écosystèmes aquatiques subarctiques de l'est du Québec, et d'autre part,
les conn aissances sur la di versité génétique des daphnies du compl exe d'espèces pulex. De
plu s, ce proj et tentait d 'évaluer les fac teurs bi otiques et abi otiques responsa bles de la
di stribu tion de ses différents génotypes à l' est de l' Amérique du nord.
Les étangs de la région de Kuujjuarapik et d 'Umiuj aq ont été caractéri sés et
l'assembl age des commun a utés zooplanctoniques décrites. Quatre fac teurs ph ys icochimiques
(température, conducti vité, altitude et COD) responsables de cet assembl age ont été identifi és.
Dans la même étude, la di versité spécifique et génétique du complexe pulex ont été décrites .
La conducti vité, le pH et la profonde ur des étangs expliquaient une grande proporti on de la
vari ati on dans la di stributi on des c lones.
Cette é tude a égal ement tes té l' hypoth èse selon laquelle les asex ués et les polypl oïdes
avaient une di stributi on plus large et occupaient des régions froides e t marginales grâce à une
plus grande fl exibilité mé tabolique et une plus grande tolérance phys iologique aux conditi ons
environn emental es extrêmes que les dipl oïdes sex ués. De ux expéri e nces de la boratoires ont
mi s en évidence que la di stributi on géographique des modes de reproduc ti on et des ni veaux
de ploïdi e ne supportait pas cette hypothèse. Des fac te urs hi storiques expliqueraient mi eux la
parthénogenèse e t la polypl oïdi e géographiques . Finalement, ce proj et a confirmé que les
c lones de daphnies sont spéciali stes et occupent des niches éco logiques différentes. Associé à
une grande di versité génétique, cela pounait ex pliquer la di stributi on cos mopolite des
daphni es du compl exe pulex.
v
TABLES DES MATIÈRES
REMERCIEMENTS .. ......... .. ...... ... .. ..... ..... .. .. ... ... ......... ..... . ..... . . . .... . ... ..... ....... .. ii
RÉSUMÉ .. ... ........ . ... .. ...... ........ .. ..... ...... ..... . .................... . ................. .......... iv
TABLE DES MATIÈRES .. . .. .... . . .. . . .. ..... .. ... . .... ...... .. ..... ... .... .. .. ... ......... ... .... ..... .. .v
LISTE DES TABLEAUX ...... ... .... .. ... .......... ... ..... ... ...................... ..... .... ....... .... .vii
LISTE DES FIGURES . ... .... ....... ........ ................... ..... .......... ... . ......... ... . . ...... .. ix
INTRODUCTION GÉNÉRALE ...... . ....... .. ................................................ .... . .... 1
CHAPITRE PREMIER
ECOLOGICAL FACTORS INFLUENCING ZOOPLANKTON ASSEMBLAGE AND
GENETIC DIVERSITY OF THE DAPHNIA PULEX COMPLEX IN SUBARCTIC
PONDS .............. .. .. .. .. ........... ... .. ...... . .......... ... ....... . . ........ .. . .............. ... . ..... 14
l.1- Rés umé (Abstract) ................. ... . .... ............. . .. ........ . ............ . ..... .... .... ...... 15
l.2- Introd ucti on ... ... ................... ... .. ... .... .. . .. ...... .. .. ...... . . ............................. 16
1.3 - Matériels et méthodes (Materials and methods) . .. .. . ....... ... . .............. .. .. ....... ....... .18
1.4- Rés ultats (Res ults) ... .......................... ... . .. ........ ..... .. ....... .... .. .......... ........ .23
l.5- Di sc ussion and conclusion ........................... ... ................. . . . ............. ... ... . .... 35
CHAPITRE 2
LOW FLEXIBlLITY OF METABOLIC CAPACITY IN SUBARCTIC AND TEMPERA TE
CYTOTYPES OF DAPHNIA PULEX .. .. . ... ... ... ... . .... ............... . ...... . .... ................ 44
2.1- Rés umé (Abstract) ... .............. ...... ... ...... . ......... .. . ............... ... ................ .. .45
2.2- Introduction .. .... .... .... .. ........ ........ . ..... ... .. ...... .. ...... ..... . ... ... ... ................ . .46
2.3- Matériels et méthodes (Materials and methods) .......... ........ .. ....... ... ............. .. ... .49
2.4- Rés ultats (Res ults) . . .... .. ..... . ... ... .. . .. ... ... . .... .. .. .... . ........... ..... ....... . .. .. .... . .... 51
2.5 - Di scussion and concl usion .. . ..... .... .. . .. .............. ........... .... ..................... .. ... .. 55
2.6- Remerciements (Acknowledgments) ...... ..... .... ....... ....... ...... . .... .. ... .... ....... ... .. 58
CHAPITRE 3
VI
DIFFERENTIAL SURVIVAL AMONG GENOTYPES OF DAPHNIA PULEX DIFFERING
IN
REPRODUCTION
MODE,
PLOIDY
LEVEL
AND
GEOGRAPHIC
ORIGIN .. ... ............ .. .... ...... . . .. .. ... .. .. .. .. ... . ...... ... ...... ..... . .... ... . .. .. . . .. ...... . .. .... 60
3.1- Rés umé (Abs tract) ............. .... ..... ....... . . ....................... .. .... .. ........... ......... 61
3.2- Introduction ... . .......................... .... ............... ... . . ........ .... ....................... .62
3.3- Matéri els et méth odes (Materials and meth ods) .................... .. ........ .. .. ............... 64
3.4- Rés ultats (Res ults) . ..... . .. . ... .. . .... . .... . .. . ..... .. ...... .. ........... . ........... ...... .. ... .... 66
3.5- Discussion and conclusion ......................... .. ... .. .. ....................... ...... .... .. ..... 74
CONCLUSION GÉNÉRALE ......... .. ....... .. ... .... .... .. ......... . ................... ........ . ... .81
BIBLIOGRAPHIE ................ ... . . ........ .... .. ............. .. .. .. ... ... ..... ... ..... . .......... .... 86
VII
LISTE DES TABLEAUX
CHAPITRE 1
Tabl eau 1
Limnological c harac teri sti cs of the ponds studied .. . .. . . ... .... . . .. .. . . . . ... ... ... .25
Tableau 2
Descripti on of clones, percentage of ponds in w hi c h eac h clone was detected,
percentage of indi viduals belonging to eac h clone in a il the survey and events of co-occ urence
with name of clones in the sam e pond ..... . .. . ...... ... .. ... ... ........... . .. .. . .. . .. ....... . . .. .... .. 30
Tableau 3
Type of pond s, number of clones which were detec ted in eac h pond, pl oidy
levels encountered in eac h pond and Simpson's index of di versi ty (l -D) . . .......... .. .......... 32
CHAPITRE 2
Tableau 1.
Mean (S. D .) sampl e mass (mg) and protein content (g protein * g tiss ue' l) in
fo ur clones of Daphnia pulex (n
= 4). PN: polypl oid clones from Nordi c region; PT: polypl oid
clone from temperate region; DN: dipl oid clone from Nordic region; DT: di p loid c lone from
te mperate region ................. .... ... ... . ............ ... .. ........... .. ... .......................... .. .. .. .... ... ..... .... 53
Tab leau 2.
Mean (S. D .) CS, ETS and LDH ac tiviti es (U*g ti ss ue,l) of fo ur Daphnia pulex
clones ex posed to di fferent pH and temperature treatm ents durin g four days ... ... ............. . 54
CHAPITRE 3
Tabl ea u 1.
Di stributi ons and reproducti ve charac teri stics of twelve clones of Daphni a
pul ex . ..... .. .... ... . . ..... . . . ... . ....... .. ..... . . . . . . . . . .... ... .......... .... ............ . ........ . . .. . . ..... 68
Tableau 2.
Effect of clone and reproduc ti on mode on surviva l ra tes after a 48 h ex position
to each treatment accordin g to Kru skal W allis test . .... .... .. ... ......... ... .. ..... ..... .. ................. .. ...... 68
Tabl eau 3.
Medi an lethal time (LT
50)
and 95 % fiducial limi ts for eac h clone a t 80 and
1500 uS .cm,l. 0 in p arenthesis mean that no mortality was obse rved untiI 48 h .................. 72
Tabl eau 4.
Median le tha! time (LT
50)
and 95 % fiduci a! limi ts for eac h clone at pH 5 and
pH 10. 0 in parenthes is mean th at no mortality was observed until 48 h .......................... ... .73
VIIl
Tablea u 5.
Medi an leth al time (LT
50)
and 95 % fiduciallimits fo r eac h c lone at 30°e. 0 in
parenthesis mean th at no morta lity was observed unti l 48 h .......... . .. .. .. ... . . ... . . ............. 74
IX
LISTE DES FIGURES
CHAPITRE 2
Figure l.
Loca ti on of the studied area ................ .... .. ......... . .. .... . ................... . . 18
Figure 2.
Tripl ot res ulting from RDA with species as response vari ab les and
environmental fac tors as explanatory vari ables .... .. ........ .... ................. ..... .............. 28
Figure 3.
Relative abundance of Daphnia species in the three ty pes of ponds ......... ... 31
Figure 4
Triplot res ul ting from RDA with clones as res ponse variab les and
environm enta l facto rs as exp lanatory vari ables .............. .............................. .......... 33
res l
CHAPITRE 4
Figure 1.
Mean (S. D .) surviva l (%) of twe lve Daphnia pu/ex clones (n
= 3) ex posed to a
48 h ac ute co nductivity .... ... .. .. ............... . ........ .. ................. . ......................................... 69
Figure 2.
Mean (S. D.) survival (%) of twe lve Daphnia pulex c lones (n
= 3) exposed to a
48 h ac ute pH ... ..... .... .... .. . . ........ . . ......... .. .. .. ..... . ........ ... ... ... . ....................... .. .70
Figure 3.
Mean (S. D .) surviva l (%) of twe lve Daphnia pulex clones (n
= 3) ex posed to a
48 h acute temperature .. . . .................... ... ........... .. .......... . ............. . .................. .71
INTRODUCTION GÉNÉRALE
Structuration d'un écosystème
La complexité des écosystèmes a toujours fasciné les écologistes. De la théorie de la
niche écologiq ue à la théorie neutre de la biodiversité et de la biogéographie, de nombreuses
hypo thèses ont tenté d'appréhender cette complexité. Bea uc o up d'attention a été portée sur la
structure, la composition (Rautio 1998; Armengol and Miracle 1999; Swadling, Pienitz et al.
2000; Rautio 2001; Cotteni e and DeMeester 2004; Yo un g and Ri essen 2005; Norlin, Bayley
et a l. 2006) ainsi que sur l'établi sseme nt (Louette, Mieke et al. 2006) e t la résilience des
comm unautés (Brock, Nielsen et al. 2003) . Les paragraphes suivants sont une desc ripti on
non-exhaustives de ces hypothèses et des récents travaux effec tués en écologie des
populations .
La composition d'espèces et la structure des communautés dépe ndent de l' interaction de
facteurs biotiques et abioti ques, co mmunéme nt c lassés sous deu x ni vea ux de processus
(S hurin 2000). Les process us locaux incluent la prédation, la compétition , l'acq uisition des
ressources et l' hétérogénéité environnementale de l'habitat, tandi s que les processus
régionaux comprennent l'extinction des populations locales e t la colonisation via la dispersion
(Cottenie and DeMeester 2004). L'importance relative de ces deux niveaux de processus
générant des différences de composition spécifique varie parmi les types d'habitats et a
suscité de nombre uses intelTogations . Des auteurs ont montré que des effets stoc has tiques
(histoires et séquences de colonisation des espèces, effets de priorité) assoc iés à des taux de
dispersion bas pouvaient déterminer le succès d' établi ssement et la composition d'espèces
durant la phase initiale
Cl an) d'assemblage des communautés zooplanctoniques (Jenkins and
Buikema 1998). Shurin (2000) a effectué une étude sur la résistance aux invasions et la limite
2
de la di spersion du zoopl ancton dans des étangs et a démontré que plus la diversité spécifique
es t grande, plus la commun auté es t résistante aux invas ions par de nouvelles espèces. Cela
suggère que les fac te urs locaux ont plus d ' importance que la di spersion dans la structure et la
composition des commun autés établies. En effet, les habitats nouvellement formés sont
coloni sés par des espèces présentes dans la région par di spersion. Les premi ères espèces à
coloni ser l' habitat sont ce lles dont la capac ité de di spersion et/ou l' abondance régionale sont
les plus élevées . Un déc lin dans le temps du nombre d' espèces coloni sant l' habitat rés ulte de
fac te urs loca ux, c ' est-à-dire que l' augmentati on d' interac ti ons bi otiques pe ut réduire le succès
d 'éta bli ssement de nouvea ux immigrants, notamme nt par exclusion compé titi ve. Louette et
co ll (2006) confirment ces de ux rés ultats en testant le succès d' établi ssement dans des
communautés de c ladocères de 1 an et 2 ans et porte nt une atte nti on toute partic uli ère sur
l'importance de la prédati on et de la compétiti on dans la déterminati on de la compositi on
d ' espèces des communautés zoopl anctoniques.
Certain s aute urs con sidèrent que ce sont les process us locaux qui sont responsables de
la structure des commun autés, alors que d 'autres aute urs pensent que ce sont les process us
régionaux. À cette du alité s'ajoute deux mouvements de pensée théorique qui di vergent dans
l' ex plication des patrons de compositi ons d'espèces. D ' un côté, la théori e de la ni che
écologique suggère que les patrons de bi odi versité sont étroiteme nt reliés a ux vari ati ons de
nombreux paramètres écologiques locaux (conditi ons environne mentales, compé titi on,
prédati on ... ). Par conséquent, la coexiste nce d'espèces es t liée à la di fférenti ati on des ni ches
écologiques de chaque espèce. De l' autre côté, la théori e neutre de Hubbell (200 1) ignore les
di ffére nces e ntre les indi vidus dans le urs réponses aux condi tions écologiques locales et
considère la dispersion comme le principal facte ur structurant les commun a utés locales,
c haque indiv idu étant équivalent. Les prédi ctions de la théorie neutre pe uvent représenter des
3
hypothèses nulles pour ex pliquer les patrons de structure de communautés (Be ll 200 1). Si les
théories de la niche écologique et neutre sembl ent s' exclure l' une de l' a utre dans leur
définition, des cas ont démontré leur possible coex istence. Récemme nt, Th ompson et
Towsend (2006) , par exemple, ont mis en évidence le po tentiel , aussi bi e n des process us de
dispersion que des conditions environnementales locales pour ex pliquer les patrons locaux de
diversité de commun autés d' in verté brés de ri vière. Une faible capacité de di spersion pour une
espèce favorisait la théorie de la niche e t une grande capacité de di spe rsion pour une autre
penchait pour la th éori e ne utre.
Enfin, la taille et la structure de l' habitat pe uvent éga lement influencer le nombre
d'espèces dans une commun auté. Bien que la théorie de la bi ogéographie insulaire soit basée
sur le fait que le nombre d 'espèces es t proporti onne l à la taille de l'île (M acArthur and Wil son
1967), il a é té démontré qu e de petits habitats peuvent présenter une grande ri c hesse
spécifique. C'es t le cas des lacs pe u profonds e t des étangs (Scheffer, Geest et al. 2006).
L'écosystème aquatique subarctique
L'environnement abiotique
Les étan gs sont des écosystè mes re lativement isolés les un s des autres et représentent
des 'îles' aq uatiques dans un paysage telTestre. Par conséquent, c haque é tang es t une parcell e
entourée d' une m atrice hos tile (o ù les individus ne pe uvent se no ulTir et se reprodui re) comme
le propose la théorie de la métapopulation (Hanski and Gilpin 1997). Les plans d'eaux de
l'Arctique e t du Subarctique sont parmi les écosystèmes qui réagiront en premier au
4
réc hauffement global de la planète (Boer, Kos ter e t al. 1990). Par conséquent, comprendre ce
qui détermine les patro ns actuels de di stributi on des espèces es t essenti el afin d' évaluer les
futurs c hangeme nts de biodi versité et de structure de communautés qui pounaient résulter des
changements climatiques. De plus, le nombre très élevé d'étangs nordi ques dans une
superficie relati vem ent petite offre une grande opportunité de rec herche écologique
expérimentale et théori que sur le ten ain . En effet, les lacs e t les étangs sont formés de
mani ère très dynamique. Année après année, le ge l e t le dége l font craquer le sol et
agrandi ssent ces fiss ures, qui se comble nt de neige en hiver. En été, la fonte des neiges et la
pluie rempli ssent les trous d 'eau (B ole n 1998). Parce qu ' il s sont pe u profonds, de petite
superficie et dans un climat rude, ces microhabitats aquatiques, qui représentent 2% de la
surface de la Ten e, subi ssent de grandes fluctu ations physico-c himiques (Rauti o 1998; Rauti o
2001 ). Il s gè lent compl ètement en hi ver tandi s que l'évaporati on peut les asséc her e n é té.
L'environnement biotique
La présence d ' in vertébrés aquatiques dans ces étangs temporaires est donc grandement
liée à le ur tolérance aux conditi ons vari ables de l' environnement (processus loca ux) et à le ur
capacité de di spersion e t de coloni sati on (processus régionaux) (Sheath 1986). La capacité de
di spersion varie selon les espèces et peut ê tre active ou passive. Cette derni ère, utilisée par le
zooplancton, es t assurée par la forma ti on d' œ ufs dormants durant le cycle de vie. Emportés
par le vent ou attac hés aux plumes d 'oisea ux aqu atiques, il s peuvent être transportés sur de
grandes di stances (M aguire 1963). La formati on de ces œ ufs perme t également de résister au
ge l et à la dessicca ti on e t de ré tab lir les popul ati ons quand les bonnes condi tions sont de
re tour au printemp s. Ce phénomène, appelé résilience, se produit après c haque perturbati on et
les communautés rep assent par tous les stades de colonisation et d 'établissement. L 'effi cacité
5
d'éclosion des œufs et le temps très court pour compléter son cyc le vital sont de gran de
importanc e. À l'échelle locale, les facteurs abiotiques qui sembl ent con élés aux patrons de
distribution du zoop lancton incluent le pH, la taille des étan gs et la proximité des autres
étangs (Swadlin g, Gibson et al. 2001 ) tandi s que les facteurs biotiques sont principalement la
biomasse chl orophyllienne, la compétiti on et la prédation (Rauti o 1998). Certains facte urs
abiotiques peuvent éga lement interagir avec des conditions biotiques; la température peut
avoir un effet négatif pour une espèce sur la capacité d'exploitation d' une ressource, par
exemple (Steiner 2004).
Le zoop lancton domine souvent dans les étangs peu profonds des hautes latitudes. On le
retrouve même à des densités très é levées (Rautio and Vincent 2006), étant favorisé par
l'absence de prédateurs vertébrés, rés ultant en une chaîne al imentaire si mplifiée, à deux
ni vea ux trophiques (Rauti o 1998). Par contre, l'assemblage de prédateurs in vertébrés j oue un
rôle important dan s les étangs. Les larves de l' insecte Chaoborus, les copépodes carni vores
tels Hete rocope et plusieurs cyc lopoïdes sont grandement responsables de la structure des
communautés zooplanctoniques. Ces prédateurs se nouni ssent de zoopl ancton herbivore tel
que copépodes ou c ladocères, qui eux , compétitionnent pour la nouniture, principalement
composée de nan oplancton et de diatomées (Rauti o 2001 ). Parallèlement, les rotifères,
composante principale du micropl ancton, broutent la production primaire et consomme la
production secondaire bactérienne (Rublee 1998). La bi omasse alga le de la colonne d' eau
d 'étangs peu profonds sembl e être une ressource insuffi sante pour le nombre élevé d'espèces
zooplanctoniques herbi vores puisqu ' une étude récente a montré que ces espèces pouvaient se
nounir substantiellement sur la biomasse algale benthique se trouvant dans le tapis mi crobien
au fond des étangs (Rauti o and Vincent 2007). Des études précédentes ont identifié six
espèces de copépodes et cinq espèces de cyclopoïdes dans 37 étan gs dans la région de
6
Kuujjuarapi k. Leptodiaptomus minutus était l' espèce dominante dans la plupart des étangs,
so uvent co-ha bitant avec d'autres espèces de copépodes (Swadlin g, Gibson et a l. 2001 ).
Également, le cladocère Daphnia middendorfiana é tait présent da ns to us les étangs étudiés
dans ce tte même région en 2006 (Rauti o and Vincent 2006 ). D 'autres études ont montré que
les cladocères du genre Daphnia dominent le zoopl ancton des é tangs nordi ques (Weider,
Hobaek et al. 1999). Ces broute urs sont principalement prédatés par les larves de l' insecte
Chaoborus (Colbourne, Crea se et a l. 1998; Ri essen and Youn g 2005). Lo uette et coll. (2006)
ont montré que les compositi ons de communautés de cladocères sont principalement
influencées par la compétiti on pour la ressource et la prédati on par Chaoborus.
Parthénogénèse, polyploïdie et répartition géographique de Daphnia pulex
Les me mbres du compl exe Daphnia pulex se reprodui sent par parthénogenèse cyc lique,
comprenant une phase sex uée sui vie d ' une phase asex uée et par parthénogenèse obli gatoire,
stri c tement asex uée (Innes, Fox et al. 2000). Cette derni ère est carac téri sée par une
descendance produite à partir d ' œ ufs non fertili sés dipl oïdes ainsi que des œufs dorm ants ou
éphippies, possédant un bagage génétique identique à celui de leur mère (O ·ease, Stanton et
al. 1989). Pour plusie urs auteurs, la transition de la parthé nogé nèse cyc lique à la
parthénogénèse obli gatoire est due à un a llè le de suppression de la méiose limité par la
feme ll e (Innes and Hebert 1988) . La propagati on de ce t allèle dans une popul ation à
parthénogénèse cyclique a pour effet de figer sa vari abilité génétique dans di vers génotypes
clonaux (Crease, Stanton et al. 1989).
Les daphni es se reprodui sant par parthénogenèse obligatoire ou asex uées, dominent
leurs congénères sex ués sous des latitudes et des altitudes élevées. Le terme parthénogenèse
7
géographique a é té ava ncé pour expliquer cette di stributi on spati ale di sj ointe entre les
organi smes sex ués e t asex ués de la même espèce (Van deI 1928).
De plus, en Arcti que, la parthénogénèse obligatoire est souvent associée à la po lypl oïdi e
(une augmentation du nombre de j eux de chrom osomes) (Dufresne and Hebert 1994). La
polypl oïdi e pe ut survenir sui te à l' uni on de gamètes non réduits (2n) avec des gamètes réduits
(n). Lorsque les uni ons de gamètes se font au sein d' une même popul ati on, on parle
d 'autopo lyploïdi e. Par contre, lorsque les uni ons se font entre gamè tes provenant de génomes
di fférents, on parl e d 'allopo lyploidi e (Otto a nd Whitton 2000). En zone tempérée, on re trouve
presqu ' exclusivement des daphnies dipl oïdes; dans le subarctique, plu sieurs clones dipl oldes
co-existent avec des c lones polypl oïdes; et dans le haut Arctique, on note une prédomin ance
de c lones polypl oïdes (Beaton and Hebert 1988) . Le changement de ni veau de pl oïdi e produit
un isolement reproducte ur qui empêche une réversion à la sexualité (Hebert, Schwartz et al.
1993) . La combin aison de de ux ou plusie urs génomes di vergents entraîne souvent un taux
élevé d ' hé térozygoti e chez les polyploïdes, ce qui pe ut les doter de nouvelles to lérances
écologiques (Dufresne and Hebert 1998) pouvant expliquer le ur grande répartiti on dans les
milieux ex trê mes (a lpins et arctiques).
Problématique de l'étude
L 'alliance de deux disciplines
De nom breuses études ont été effectuées sur la biologie des commun autés et l'écologie
du zooplancton (Rubl ee 1998; Armengo l and Mirac le 1999 ; Rautio 2001 ; Swadlin g, Gibson
et al. 200 1; Steiner 2004 ; Ri essen and Youn g 2005 ; Norlin , Bay ley et al. 2006 ; Scheffer,
8
Geest et al. 2006). Également, de nombreuses études ont été effectuées sur la di vers ité
génétique de plusieurs espèces d' invertébrés aquatiques (Spaak 1997; Wilson , Sunnucks et al.
1999; Innes, Fox et al. 2000; Crease 200 1; Hebert and Finston 2001 ; Palsson 200 1; Winsor
and Innes 2002; Michels, Audenaert et al. 2003; Weider and Hobaek 2003; Haag 2005;
Paland, Colbourne et al. 2005 ). Par contre, rares sont les rec herches qui allient ces disciplines
so us un même obj ectif (Weider, Makino et al. 2005). Pourtant, les théories sur la diversité en
génétique des populations et en écologie des communautés partagent plusieurs similarités
frappantes . Ces deux di sciplines montrent un intérêt commun dans l'explicati on des nombres
et des fréquences rel ati ves des variances biologiques trouvées en nature. Tandis qu'en
génétique des populations, ces variances sont les génotypes ou les all èles, en écologie des
communautés, ce sont les es pèces. Ve llend a fortem ent contribué à la mi se en évidence d'un e
con élation positi ve entre la di versité spécifique et la di versité génétique. En 2003, il teste ce
qu ' il nomme le SGDC, conélation de la diversité spécifique-gé nétique, sur 14 bases de
données compilées à partir de la littérature sur des oisea ux, des reptiles, des mammifères et
des plantes, dans une grande vari été d' archipels. La con é lati on étai t généra lement positi ve.
En 2004, il trouve que la variation génétique de l' herbe Trillium grandiflorul11 es t moin s
importante dan s les forêts secondaires, poussant sur des tenes agricoles abandonnées,
comparativement aux forêts primaires, non perturbées. Également, la diversité spécifique dans
les forêts secondaires était faible. Les facteurs expliquant le plus cette conélation positi ve
sont la surface et l'histoire d' utili sati on de l' habitat combinées avec des variations autant dans
la taille de la population (une espèce, en particuli er), que dan s celle de la communauté (toutes
les espèces appartenant au même taxon) (Vellend 2003; Vellend 2004). Ces deux ni veaux de
diversité sont modulés par les mêmes facteurs (déri ve des communautés ou génique,
migration des indi vidus ou des allèles, sélection sur les deux ni veaux de diversité et
spéciation/mutati on). Ce projet de maîtri se vise donc à intégrer ces deux di sciplines dan s
9
l' étude des facte urs qui influencent la di versité des communautés de zoopl ancton, la di versité
interspécifique des daphnies du complexe D. pulex ainsi que la di versité intraspécifique o u
clonale.
La dive rsité clona le
La di versité génétique des daphni es a été beaucoup étudi ée tant dans l'Arctique
(Dufresne and He bert 1995 ; W e ider, H obae k e t al. 1999), que dans les régions tempérées
d ' Amérique du no rd (H ann 1996; He bert and Finston 2001 ) ain si qu 'en Argentine
(A damowicz, Gregory e t al. 2002). La di versité c lo na le est plus grande dans l' Arctique que
dans les régions tempérées (e nviron 4,5 c lones pour moins de trois par é tang, respec ti vement)
(Dufresne and He bert 1995). Égal ement, les popul ati ons de la région de Churc hill au
M anitoba (Canada) à l' oues t de la bai e d' Hudson sont très di versifiées (Wil so n and Hebert
1993). Cependant, a uc un e informati on sur la di versité cl ona le des daphni es de la région à l'est
de la baie d' Hudson, auto ur de Kuujjuarapik (Nun avik ) n'est di sponibl e. Lors du pléistocène,
les glaces couvrant la qu asi-tota lité du continent nord-am éricain se sont re tirées permettant
aux espèces subsista nt dans le refu ge glaciaire de la Bérin gie (Al as ka) de se propager à partir
du nord-oues t vers le reste du continent. Kuujjuarapik étant plus é loignée du refu ge glaciaire
de la Bérin gie (A las ka) que C hurchill , on peut prédire que la di versité c lo nale devrait ê tre plus
faible. De plus, la nature insula ire des é tangs permet des différenti ati ons génétiques e t des
adaptati ons locales entre les popul ati ons (DeM eester 1996). Vanoverbe ke et DeMees ter
(2005 ) ont mon tré qu ' il n 'y avait pas de re lati on significati ve entre la di stance géographique
e t la différe nti ati on gé né tique de populatio ns de Daphnia magna séparées de 100 m à 500 km .
Cec i était expliqué principa lement par le fait que les populati ons proc hes étaient déjà
hautement différenciées géné tiquement. Les popul ati ons avec pe u de c lones étaient les plus
divergentes. Cette é tude vise à combler les lacunes concernant la di versité clonal e des
10
daphnies de la région subarctique de l' est de la baie d ' Hudson et à tenter d ' expliquer
l' infl uence re lati ve des processus loca ux et régionaux sur la structure des commun autés de
zooplancton.
Répartition géog raphique des clones de Daphnia pulex
Si la ré partiti on des clones en Amérique du nord a été bie n décrite, les facteurs
responsables de la di stributi on des modes de reproduction et de la po lypl oïdie so nt moins bien
compri s. Peu d'études ont été conduites afin d 'examiner les tolérances ph ys iologiques et
écologiques dans un taxon naturel où coexistent des indi vidus sex ués, asex ués, dipl oïdes et
polypl oïdes . Le compl exe d 'espèce Daphnia pulex représente une be ll e opportunité de
rec herche sur cette problé matique pui sque l'o n retrouve des dipl oïdes e t des po lypl oïdes e n
sympatrie dans la région du subarctique e t des sex ués e t des asex ués e n sympatrie, notament
dans la région des Grands Lacs en Amérique du nord . Ses membres couvrent également un
grand gradi ent
latitudin al e t sont présents dans des
habitats dont les conditi ons
environnemental es peuvent être ex trêmement di vergentes . Enfin, la facilité de mi se en c ulture
et de manipul ati on fo nt de Daphnia un organi sme modèle en écologie expérimentale et
perme ttent
de
tes ter
des
hypothèses
sur
la
di stribution
comparati vement aux sex ués.
Objectifs
Cornmunautés zooplanctoniques des étangs subarctiques
des
asex ués/polyploïdes
11
Le premier objec tif de ce proj et de maîtrise es t de carac téri ser les étangs subarctiques et
d'en répertorier la biodiversité des macroin vertébrés dan s la région de Kuujju arapik et
Umiuj aq. Les différe ntes espèces d' in vertébrés (larves d' in sectes et cru stacés) présents dans
trois types d'étangs (roc heux, toundrique et thermokarst) ont été identifiées et qu antifiées afin
de décrire les commun autés zooplanctoniques. Également, cet obj ec tif vise à déterminer
l' importance re lati ve des process us locaux et régionaux influençant la di stribution des
différents taxons zoopl anctoniques. Les fac teurs à l'étude sont l'abondance des différentes
espèces de prédateurs et de compétiteurs, la température, le pH, la salinité (mes ures de
conductivité de l'eau), la bi om asse chlorophyllienne, le carbone organique di sso us (DOC), les
carac téris tiques morph ométriques des étangs, la transparence de l' eau ain si que le
pourcentage de végétati on entourant les étangs .
Répartition géog raphique et diversité génétique des daphnies du compLexe D . puLex
Le deuxi ème obj ectif tente d'estimer la di stributi on et la diversité génétique des
daphnies du complexe D. puLex. Dans un premier temps, les commun autés de daphnies de ce
compl exe prése ntes à l'été 2007 ont été déc rites dan s la région de Kuujju arapik et Umiuj aq.
De plus, l' influence de divers facteurs bi otiques et abiotiques sur les différentes espèces du
compl exe ainsi que sur les clones de ces espèces ont été testées lors de la campagne
d'échantillonnage. Dans un deuxième temps, deux expéri ences de laboratoire ont été
effectuées afin de comprendre la répartition géographique de différents génotypes de Daphnia
puLex. Dans la première ex péri ence, l'activité de trois enzymes impliquées dans les
métaboli smes aérobique e t anaérobique (c itrate synthase, système de transport des électrons et
lactate dés hydrogenase) o n été mesurées sur quatre clones de daphnies qui diffèrent de niveau
de ploïdie et d'origine géographique après une exposition de quatre j ours à di vers traitements
12
de température et de pH. Une plus grande flexi bilité métabolique c hez les polypl oïdes pourrai t
expliq uer leur succès dans les régions froides et ex trê mes de la pl anè te. Dans la de uxième
ex péri ence, la tolérance de c lones dipl oïdes, tripl oïdes, sex ués, asex ués e t iss us de régions
tempérées et nordiques a été comparée so us une te mpérature élevée, un pH bas ique, un pH
ac ide, une faible conducti vité e t une conducti vité élevée. Les di ffé rences de to lérances à ces
conditions environnementa les poulTaient expliquer la différence de répartiti on des di fférents
génotypes de daphnies.
13
14
CHAPITRE 1
Ecological factors influencing zooplankton assemblage and genetic diversity of the
Daphnia pulex complex in subarctic ponds
Caroline Jose and France Dufresne *
Laboratoire de Bi ologie Évolu tive, Département de Bi ologie, U ni versité du Québec à
Rimouski ,
300 Allée des Ursulines, Rim ouski , Québec, Ca nada G5L 3A l
* Correspondin g auth or. Tel. : 141 8723 1986 # 1438; fax:
141 87241 849 .
E-mail address: france-dufresne@ ug ar.gc.ca
15
1.1
Abstract
Ecological functionin g of subarcti c ponds and the re lati onship be tween environm ental
fac tors and zooplankton c ommunity, spec ies di versity and clona I di versity of the comp lex
Daphnia pulex has been in ves tigated in the Kuujju arapik and U miujaq region. Ponds showed
great vari ati on in environmenta l conditi ons and zoopl ankton abundance and struc ture. Am ong
ail the studi ed vari ab les, tempe rature, conductivity, altitude, and DOC accounted fo r 69 .7 %
of the vari ati on in zoopl ankton di stributi o n. Copepods and Daphnia toget her domin ated a
hi gh proportion of the ponds. A t the in terspecific level, depth intluenced the di fferent
dis tri bution of D. pulicaria and D. pulex, the latter prefelTing shall ow waterbodi es, thu s
exclu din g in tercompetiti on. Clona I di versity of the p ulex complex was 1. 86 clones pel' pond.
Conductivity, pH a nd depth acco unted for a great percentage of the variati on in c lones
distribution. Tripl oid c lones were res tri cted to low condu ctivity water bodi es whil e dipl oid
c lones of D. pulex were fo und mos tl y in roc k ponds th at ex hibited re lati ve ly hi gh
conduc ti vities . C lo nes inside eac h spec ies had different ecological preferences and th us could
be viewed as specia li sts. The large number of clones that have originated polyph yleti ca ll y in
thi s compl ex thu s contribute to the wide di stributi on and ecological tl ex ibility of thi s spec ies.
Keywords: S ubarcti c ponds, zoopl ankton, environmenta l fac tors, Daphnia pulex, c lonai
di versity
16
1.2
Introduction
Shall ow ponds are a major feature of high latitude landscape. As they are re lati ve ly
isolated habitats, abundant, and have a spati al patchiness, they offe r a grea t opportun ity to
study fundamental ecological processes in natural systems. Their species compositi on and
community struc ture fa ll under two sca les of processes (Shurin 2000). Competiti on,
predati on, pH and conductivity are included
111
local processes whereas di spersion and
coloni za ti on act at a regional scale (Cottenie and DeMeester 2004). Great flu ctu ati ons of
environme nta l conditi ons ac t yearl y and seasonall y on these ponds. They can dry in summer
and freeze dow n to the bottom in winter. Moreover, the sunny season is very short for the
ph ytoplankton growth and the qu antity and quality of the primary producti on can have
substanti a l reperc ussions on the di versity and abundance of tax a occ ulTin g in nex t trophic
levels. In vertebra te spec ies occ urrence in the se temporary or semi -temporary ponds is greatl y
influe nced by the ir tolerance towards vari a ble environmental conditi ons and di spersion and/or
coloni za ti on capac i ti es. Many zooplankton speci es possess developmental stages (res tin g
eggs, ephippia) resistant to drou ght and freezin g. They can therefore di sperse by th e way of
wind, birds or hum an and coloni ze or re-establish when environm ental conditi ons are
favorable again . Zoopl ankton ca n be found in hi gh densities in shall ow subarctic ponds
because of the lack of vertebrate predators but only few species are present in a sin gle pond
(Rauti o and Vince nt 2006). In fact, invertebrate predators pl ay a sub stanti al rol e in structuring
zooplankton communities . Larval stage of the phamton midge Chaoborus and sorne
cyclopoids feed on grazers like microcrustaceans. C ladocerans and copepods compe te for
phytopl ankton (Ra uti o 2001 ) and roti fers graze primary and secondary produc ti on (Rubl ee
1998). A study reported Il species of copepods in the Kuujj arapik region although species
di versity ranged from 0 to 4 species per la ke (Swadling, Gibson e t al. 2001 ). Thi s study
17
showed that conducti vity, pH and di ssolved orga mc carbon concentrati ons influence
copepods di stributi on .
Members of the Daphnia pulex compl ex are abundant in subarcti c ponds. They
reproduce by cyc lic parthenogenesi s, an asexual phase foll owed by a sex ual phase or by
obli ga te parthenogenesis, stri ctl y asex ual (Innes, Fox et al. 2000). The res ultin g clones are
geneti call y simil ar to their mother but numerous different clones have been identifi ed in
North Ameri ca. The transiti on to asex uality is due to a sex-depe ndant meiosis-s uppressor
allele that is spread in popul ati ons and fi xes geneti c di versity in multiple clonaI genotypes.
The predomin ant reproducti ve mode of Daphnia pulex in north Canada is obli gate
parthenogenes is (He bert, Sc hwartz et a l. 1993). Moreover, polypl oidy often occ urs in hi gh
latitudes. In arcti c regions, clones are polyploid whil e they are dipl oid in temperate regions
(Dufresne and He be rt 1995). In the region of Churchill (Manitoba), cl onaI di stributi on of the
Daphnia pulex compl ex has been cOITelated with salinity gradi ents th at are a functi on of
di stance and locati on of pond with respec t to Hudson Bay (W eider and He bert 1987). Among
numerous clones in thi s region, 2 domin ated the ponds. Predator occ ulTence in these habitats
determined presence of one and ab sence of the other clone (Wil so n and He bert 1993). No
info rmati on on c lona I di ver sity and di stributi on is avail able for sites in eas te rn Hudson Bay.
T he obj ec tive of thi s study was 1) to record and qu antify biodi versity of macroin verte brates of
the subarctic sha ll ow ponds of northern Québec and 2) to examine the influence of abi oti c and
bioti c factors on zoopl ankton di stribution, then more preci sely on species of the Daphnia
pulex complex di stributi on and finall y on the geneti c or clonaI di stributi on of the latter species
with a foc us on cl onai di versity. As subarcti c and arcti c ponds ecosys tem s will li ke ly be the
first to reac t to global warm ing (Boer, Kos ter et al. 1990), unders tandin g actual biodi versity is
necessary to assess future shi fts due to climatic changes. Thi s is the first study to compare
zooplankton biodi versity and limnological data among the three pond form ations encountered
18
in subarctic region , that are rock ponds, tundra ponds a nd thermokarst ponds, described in the
nex t sec ti on. A iso this study provides a first step to understand the ecological functioning of
subarc tic ponds through the re lationships between environmenta l fac tors and zoop lankton
community and to assess the relation ship be tween spec ies diversity and clonai diversity in
Daphnia.
1.3
Material and methods
1.3.1 Site description and sampling
Figure 1
Locati on of the studi ed area.
19
Heterogene ity of the forest-tundra region geomorphology res ults in three pond
forma tions. We named rock ponds th ose fo und on the carbonate-de rived bedroc k along th e
coast, tundra ponds those fo und inland on the Precambri an granite bedrock and thermokarsts
those fo und on peatl ands. Perm afrost aggregati on rai ses the peatl and surface and creates two
major features: palsas and peat plateau. Thawing of permafrost creates therm okars ti c ponds at
the bo ttom of palsas or on peat plateau. Vegetati on vari es greatl y among these types of ponds,
with essentiall y no or li ttle vegetati on in roc k bluffs, small pruce trees, lic he ns and mosses in
tundra landscape an d Sphagum and mosses in peatl ands. Zooplankton were sampl ed from the
e nd of june to mid-j ul y 2007. The main sampling locati on was in the Kuujjuarapik region
(Nun avik) (55 °17'N, 7r45' W ) on both side of the Great Whale Ri ver. Two rock bluffs, two
tundra sites on the north of the ri ver and th ermokarst ponds on the south of the ri ver were
sampl ed. F urther samples were coll ec ted near the vill age of Umiujaq, on the tree line, on a
rock bluff (56°33'N, 76°33' W ) and in a palsa fi e ld inl and (56°36 ' N, 76°12' W ). Some ponds
aro und Kuujjuarapik were sampled twice (at two wee ks interval) to have an idea of temporal
vari a bility within the samplin g peri od of community structure and environm enta l conditi ons
of the ponds. Ponds sam pIed in this study were shall ow (less th an 1 m depth) and small
(s urface area between 0.5 and 348 m
2
).
Water temperature, pH, conduc ti vity and tota l
d issolved solids (TDS ) were measured using CyberScan PC300 mul timeter probes imme rged
in th e 30 fi rst centime tres from the surface water. W ater samples were collected for the
determination of the concentrati ons of di ssolved organic carbon, chl oroph yll a and
phaeopigments. Filtrati on apparatus and samplin g bottl es were ac id-cleaned (HC I 10%).
Water samples used to DOC determinati on were filters onto pre-combusted 25 mm diameter
GFIF W hatm an fi lters (nominal pore size 0.7 /lm ). Filtrates were stored in glass tubes pre-
was hed accordin g to Burdige & Homstead (1994) me thod and ac idified with H 3P04 (25%) to
a fi nal pH of 2. 0. DOC concentrati ons were finall y obtained by hi gh temperature oxydati on
20
using a Shimatzu TOC ana lyser 5000A fo llowing White head and coll. (2000) method. Water
samples for the determination of chlorophyll a and phaeopigment were fi ltered onto 25 mm
diame ter GFIF Whatman filte rs (pore size 0 .7 /lm). Filters were then stored at - 80 oC until
analyses. Chlorophyll a and phaeopigment were determined by f1u orome try, following the
method developed by UNESCO (1994) . Maximun depth , width and len gth of the ponds were
meas ured and coordin ates and altitude were noted. Water color was evalu ated in the sampling
container. We noted nine different colorati ons of water from crystal clear to brown opaq ue
with a graduation of ye ll ows and browns and then classed the nominal colors in numbers
where 0
= crys tal
brown , 6
c lear, 1
= li ght ye ll ow, 2 = yellow, 3 = dark yell ow, 4 = li ght brown , 5 =
= dark brow n, 7 = very dark brown, 8 = brown opaque.
Zooplankton sa mpl es were collected with a 7 L samplin g container (or a IL hand
pitcher for thenn okarst ponds because of th e harsh accessibility of the ponds) and filtered on a
104 /lm mesh size net to have a qu antitative sample. Volume of filtered water varied between
1 Land 70 L. In order to have a sampl e representative of the overall pl anktoni c community,
the container was held sec ured on a one-meter stick to allow sampling in the middle of large
ponds and samples were collected at several points dependin g on th e spa ti a l variability of the
pond. Between one to ten sampi es were collected per pond, depending on the size, the
accessi bility of the pond and the zooplankton abundance and then pooled . Zooplankton was
tran sfened to borosilicate tubes of 50 mL and preserved with ethan ol 95 % until later counting
and identification with a microscope . Zooplankton assemblages were characterized by pl ac ing
taxa into seventeen categories. Cladocerans were identifi ed to the species level except for the
families of chydoridea, bosminidae and macrothrycidea. Ail daphnids belonging to the D.
pulex group were pooled. Damaged cladoceran were classified into cladoceran spp. Copepods
were c lass ifie d as harpac ticoids, calanoids a nd cyclopoids, except Hesperodiaptomus arcticus
21
that has been previously identified as a potenti al predator of D. pulex. Some damaged
copepods were c lassified into copepod spp. The taxa ostracods and the in sect Notonecta were
incJ uded in a separate c lass . Insect larvae (Ephemeroptera, dyti cidea, diptera, plecoptera,
un known larvaes and mosquito pupaes) were pooled into one class except for the larvae of the
phantom midge Chaoborus whic h were pl aced a single c lass as it is an important predator of
D. pulex.
1.3.2
Inter- and intraspecific divers ity of Daphnia pulex
T wenty indi viduals of the Daphnia pulex compl ex from eac h pond were randomnl y
chosen to assess the genetic di versity of thi s compl ex . lndividuals were ge notyped at six
microsatellite loc i described by Colbourne et al. (2004). The loci used were Dpu45, Dpu502,
Dpu1 83, Dpu30, Dpu6 and Dpu1 2. 2. FOI·ward primers were end-Iabe led with flu orescent dyes
FAM or TET (Life Tec hn ologies, Uni v. of Oklahoma, USA). Condi tions for PCR were as
desc ribed in Colbourn e et al. (2004). PCR products were e lec trophoresed on 6%
polyacrylamide ge ls and scanned with a FMBio III scanner (Hitac hi , Tokyo, Japan). A ll ele
sizes were assigned using a Fluorescent Ladder (Promega, USA). In order to assign clones
di scriminated in the microsatellite an alysis to their species, a fragment (»850 bp) including
part of the mitoc hondrial gene codin g for the NADH dehydrogenase subunit 5 (ND5 ) was
amplified by PCR following meth ods described in Colbourne et al. (1 998) and Weider et al.
(1999) and usin g pri mers DpuND5a and DpuND5b (Colbourne, Crease et al. 1998; Weider
and Hobaek 2003). Further seq uencing of thi s gene revea led lineages of the Daphnia pulex
complex fo und in this study.
1.3.3
Statistical analyses
22
Re lationships between the 10g lO (x+ l ) transformed abundance of the zooplankton
groups described above and potential explanatory variabl es were analysed usin g redundancy
analysis (RDA), a form of direct gradient analysis with CANOCO (TerBraak 1994). A total of
eleven environm ental meas ures were entered as explanatory variables in the RD A. These
included a ltitude, temperature, pH, conducti vity, c hl orophy ll a, phaeopigment, DOC, de pth,
water color, surface area and perce ntage of vege tati on sUlToundin g the ponds. Multi vari ate
norm a lity of data was tested in PAST vers. 1.81 with a KU1'tos is test. Some vari abl es
including pH, conducti vity, c hl oroph yll a, phaeopigment and DOC had to be Log-transformed
to assess multin orma lity of the data set. Longitude and latitude had to be excluded from the
analys is as thei r infl ati on factors were hi gh (> 20). The type of pond (roc k bl uff pond, tundra
pond and thennokarst) was decomposed in three dummy variabl es and added to th e mode l as
passive vari ab les. The forward selec ti on opti on in RDA was used to determine the minimum
number of e xplanatory variables that cou Id explain statiscicall y signifi cant Cp :S 0.05)
proporti ons of vari a ti on in the zooplankton data. The significance of the forward selection
variables was assessed using Monte Carlo permutati on tests (with 499 unrestricted
permutations).
Another RDA was conducted in order to examine re lati onships between the square root
transformed relative abundance of the clones of D. pulex and the same explanatory variabl es
as the previous analysis. Three vari ables were added in the analysis to show interac ti ons
between the clones and thei r principal predators. These are 10glO (x +l ) Chaoborus, 10g lO
(x+ 1) Hesperodiaptomus arcticus and [og lO (x+ 1) notonec ta abundances . Once again , the
fo rward selection option was used. In addition to the type of pond, five passive variables
23
describing the relati ve abundance of the species of the D. pulex compl ex were added in the
RDA tripl ot.
T he numbe r of genotypes, ie clonaI richness and the Simpson' s index of di versity were
calculated for eac h pond. Simpson' s index of di versity is an index of domin ance and has
va lues ranging from 0 to 1; 0
= community
domin ated by a sin gle c lone, 1
= community
equall y represented by ail the clones.
1.4
Results
T able l summ ari zes the c harac teri stics of the fort y-one ponds used in thi s study. A Il
other ponds were exc luded from the ana lyses because of incompl ete data. Pond s that were
sampl ed twi ce showed great simi lariti es in zoop lankton compositi on, clonai di versity and
environmental conditi ons and' onl y th e data of th e first sampling occasion of these ponds were
included in the analyses . O verall , the ponds vari ed greatl y for ail meas ured parameters. The
pH of the studi ed ponds ranged from acidic (6.0) to alkaline (8. 7) with a mean va lue of 7 .2.
Sorne excepti onall y hi gh values of DOC were found in therm okarsts th at may be due to the
large am ount of Sphagnum-m oss in decompositi on in these ponds. The maximum DOC
conce ntrati on was fo und in rock pond 13 (49 .75 mg L-
1
).
Thi s very hi gh va lue can be
expl ained by the OCC UITence of wings and a little of meat of a dead bird at the bottom of the
pond th at could ha ve increased th e DOC content. Conductivity had a wide range of va lues
(from 14 to 28800 p,S cm- ) and is very different between the three types of ponds (2887 .9 p,S
I
cm-I in rock bluff ponds, 130.1 p,S cm-I in thermokarsts and 47 .1 p,S cm-I in tundra ponds) .
These va lues dropped quickl y as ponds were far away from Hudson ' s Bay. Chl oroph yll a
vari ed considerabl y, rangin g from 0 .6 j.L glL in the tundra pond 18 to 15 .80 p, gIL in the
24
therm okarsts 19 and 20 . The number of zooplankton indi vidual s per m-3 showed also great
vari a ti on. The max imum va lue was found in the very small rock pond 13
with a surface area of 1.7 m
3
)
the same pond that had exceptional va lue of DOC. Daphnia and
copepods together dominated the zooplankton community of 29 ponds
zooplank ton .).
Cl 427 000 ind. m-3
(~
75% of the total
25
Table 1
Limnological characteristics of the ponds studied
No
pond
Long
Lat
A It
(m)
Area
(m 2)
Depth
(cm)
Tem p
(OC)
Cond
I
(JlS cm- )
pH
DOC
(mg
L-I )
::l
.D
1
2
3
4
5
6
7
8
9
10
-'<:
Il
77. 93
77.93
77.95
77.94
77.97
77.84
77.84
77.84
77.83
77.97
77.97
77.98
77.98
77.98
76.67
76.67
77.93
77.97
55.49
55.49
55.48
55.48
55.47
55.50
55.51
55.51
55.50
55.47
55.47
55.46
55.47
55.47
56.69
56.69
55.47
55.47
10.1
10. 1
2.7
2.7
3.0
8.8
8.2
9.1
10.4
2.7
4.0
0_9
2.4
2.7
15.2
0.0
5.8
2.4
5.6
(4.2)
6.5
80.2
12.5
26.0
7.7
56.0
6. 1
2.9
5.6
1.2
0.5
2.3
1.7
2.6
32.5
5.2
0.8
27. 1
15.4
(2 1. 9)
4.5
40
5
9
21
23
27
28
24
21
18
23
7
21
22
25
18
28
20.3
(9. 1)
11. 1
9.7
19. 1
16.4
15.6
11.7
12. 1
10.3
10.7
13
13.2
15.7
15
14. 1
8.4
9.7
12
12.7
(2.8)
169.8
159.6
287.0
3 15.0
1375.0
58.4
52.8
48.7
55.3
2620.0
1573.0
28800.0
5680.0
2750 .0
76.2
202.0
350.0
7410.0
2887.9
(6798.3)
7.6
8.6
8.7
8.8
8.7
6.6
6.0
6.1
6.5
7.3
7.6
8.2
7.2
7.9
7.2
7.1
7.7
7.4
7.5
(0.9)
13.7
13.1
18.4
14.2
13.0
5.4
12.2
8.3
9.3
25.8
31.2
12.4
49 .7
14.8
17.4
14.5
9.6
5.0
16.0
( 10.6)
77.89
77.89
76.46
76.45
76.45
76.44
76.46
55.36
55.36
56.77
56.79
56.79
56.77
56.77
148.7
149.0
180.7
186.2
185 .3
2 11.8
180.7
177.5
(22.3)
55. 1
37.8
166.7
86.7
95.3
3.5
3.5
64. 1
(57 .9)
90
90
30
60
80
8
8
52.3
(36 .7)
18
18
14.7
15.5
15.6
15
14.6
15.9
( 1.5)
224.0
337.0
125.5
24.7
14.0
88.9
96.8
130.1
( 114.8)
6.6
6.6
7.3
6.0
6.2
7. 0
7.2
6.7
(0.5)
35.7
37 .5
22.5
15.9
1 1.0
12. 1
16.8
21.7
(l0.9)
77.76
77.78
77.78
77.74
77.77
77.79
77.79
77.80
77.85
77.88
77.88
77.84
77.83
77.93
77.93
77.84
55.43
55.44
55.44
55.44
55.44
55.44
55.44
55.3 1
55.29
55.32
55.30
55 .54
55.54
55.5 1
55.51
55.30
Mea n
(S. D.)
36.6
60.4
57.9
46.0
55 .2
52.1
48.8
60.4
67. 1
65.8
68.6
49 .7
49 .7
91.7
91.7
94.2
62.2
(17.2)
6 1.5
62. 1
1.9
27.3
47.3
57.2
45.0
78 .1
26.3
24.6
89.0
120.0
348.2
62.4
28 .5
5.6
67.8
(80.9)
30
52
33
48
37
28
45
76
52
30
21
48
44
57
42
30
42. 1
(13.8)
8.4
8.0
7.5
9.3
10.4
1l. 2
9.7
7.2
10.4
10.1
9.7
17. 1
16.4
14.9
15
10
11.0
(3.2)
32.7
36.4
38.0
58.5
43.8
47 .2
41. 6
37 .6
45.0
33.8
20.8
52.4
56.4
64. 1
65 .8
80.0
47.1
(15.0)
7.8
7.7
6.9
7.6
7.5
7.3
7.4
7.5
6.5
6.5
6.5
7.4
7.4
7.3
7.2
6.5
7.2
(0.4)
8.2
13.9
7.6
10. 1
9.8
15.9
12.5
6.9
5.2
9.2
9.0
9.6
10.0
5.5
5.2
18.1
9.8
(3.7)
Mi n
Max
Mean
0
211.8
57.1
0.5
348.2
44.2
7
90
34.2
7.2
19. 1
12.6
14.0
28800
1308.5
6.0
8.8
7.2
5.0
49.7
14.5
Type
of
pond
<.;....
<.;....
u
0
A:::
12
13
14
15
16
17
18
Mean
(S .D.)
r;
""0
-'<:
E
....
0
.<:
f-
e
"'Cl
c:
::l
f-
Total
19
20
21
22
23
24
25
Mean
(S. D.)
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
Il
26
End of Table 1
Limnological characteri stics of the ponds studied (copep.
Daphn. = daphnia and zoopl.
T ype
of
pond
''-
:J
::0
~
u
0
Cl:::
E!
~
~
0
E
....
Q)
~
e
"0
<::
::l
f-.
Tota l
No
po nd
2
3
4
5
6
7
8
9
10
II
12
13
14
15
16
17
18
Mean
(S.D.)
Chi
a
(Jl g
4.6
1.2
2.8
5.3
1.5
2.8
12.1
2.5
1.8
2.1
4.0
7. 1
5.8
6.5
0. 8
0.9
1.3
1.2
3.6
(2.9)
Phaeo
(Jlg L1)
= zoop lankton)
Wate r
color
%
vegetation
%
Copep
%
Daphn.
Total
Zoop l.
(ind m-3)
2.3
0.5
1.6
1.7
0.4
3.3
8.4
8.8
3.8
3.2
7.0
3.6
3.4
2.3
0.7
1. 8
2.9
1.3
3.2
(2.5)
2
0
3
7
7
7
2
8
2
2
2
3
2
4
3
3. 2
(2.6)
75
70
85
40
0
10
50
10
0
0
0
0
0
0
85
15
10
0
25
(32.9)
2l.l
90.2
52.5
32.7
5.9
100.0
89.6
82. 1
73.3
34.7
62.5
0.0
1.0
9.3
8.2
1.6
28.3
2.2
38.6
(35 .8)
78. 0
8. 5
21.5
7.3
74.5
0.0
3.9
13.0
21.1
65.0
33 .6
0.3
44.9
9.5
91.5
0.0
38.2
80.8
32.9
(3 1. 8)
41402
43991
25286
7857
242 9
439682
11000
208843
12857
1306 12
43429
23286
1426857
3373 15
443 75
50833
44857
3577 1
162815
(337939)
5
5
8
8
1
8
8
6.7
(3. 1)
100
100
100
100
100
100
100
100
(0.0)
0.9
70.2
50.0
96.3
93.9
16.1
1.3
47 .0
(41.5)
92.7
0.7
0.0
0.0
0.0
6 1. 3
97.4
36.9
(46 . 1)
536000
305000
5875
15571
11000
5167
33778
130342
(209350)
19
20
21
22
23
24
25
Mea n
(S.D .)
15.8
15 .8
6. 1
3.5
3.4
7.5
0.8
7. 6
(6.0)
23.6
23 .6
2.7
2.9
2.9
6.1
3. 0
9.3
(9.9)
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
Mean
(S.D.)
9.5
3.7
2.8
2.4
1.2
2.2
2.2
1.5
1.6
2.5
2.9
2.5
0.6
2.0
1.6
2.0
2.6
(2.0)
8.4
1.7
6.6
0.8
0.8
2. 1
1.7
1.3
1.9
2.4
2.5
\.9
0.4
0.9
0.9
2.4
2.3
(2.2)
3
3
6
1
2
5
5
2
2
3
2
2
3
3
2
3
2.9
(1 .3)
5
50
0
90
50
90
50
0
100
100
80
100
100
85
100
80
67. 5
(37.3)
47.0
85 . 1
65 .2
6.7
12.5
6.3
27.6
9.1
17. 1
10.8
0.0
58.5
20.9
2.2
0.2
11.2
23.8
(26.0)
51.3
10.9
21.7
82.2
87.4
90.0
72.3
89.9
78.8
85.0
20.0
17.0
0.0
86.2
65.3
67.4
57.8
(32.5)
3286
63918
120356
3214
20237
100571
33883
2357
4857
59643
571
1514
1229
10343
155 36
3 179
27796
(38 170)
Min
Max
Mean
0.6
15 .8
5.0
0.4
23.6
3.9
0
8
3.6
0
100
54.4
0.0
100.0
34.3
0.0
92.7
43.1
57 1
1426857
104580
= copepods;
27
Zooplankton-environment relationship
The influence of Il environm ental variables on the di stribution of zooplankton groups
was assessed using RDA (Fi g 2). The first two RDA axes, with eigenvalues of 0. 189 and
0.066 for axis 1 and 2, respec ti vely, accounted for 25.5 % of the vari ati on in species data.
However, RDA axes l and 2 expl ained a hi gh proporti on (86 %) of the vari ati on in the
species-environment re lati onship . The species-environm ent con elati ons for ax is l (0.762) and
axis 2 (0.787) were hi gh, indicatin g a reasonably strong relationship between the zooplankton
group s and the e nvironme ntal vari ables . With fOl'ward selecti on and Monte Carl o permutati on
tes ts RDA identifi ed a sub set of fo ur variabl es th at explained signifi cant (p :'S 0.05 ) proporti on
of the vari ati on in the species data. These were temperature, conductivity, altitude and DOC
whi ch accounted for 69 .7 % of the vari ati on in zoopl ankton di stributi on. The tripl ot presented
in Fi gure 2 shows c lear di vergence in community structure among the three types of ponds.
For exampl e, Daphnia seemed strongly con elated with the abundance of its predator
HesperodiaptOinus arcticus
111
rock ponds and inversely con elated with Holopedium
gibberwn. Tundra ponds showed less di versity in species compositi on with onl y Polyphemus
pedicullus and Holopedium gibberum being positi vely con elated with thi s type of ponds.
Moreover tundra ponds had low DOC concentration s and low values of conducti vity.
Therm okarsts were found in hi gher altitude and were inhabited by cyclopoids, Chaoborus,
Scapholeberis spp ., c hydoridae and ostrac ods. Rock ponds were less di scrimina ted by
environmenta l vari a bles as the ranges of temperature, DOC and conduc tivity were wide. They
supported the cohabita ti on of calanoids, harpac ticoids, Daphnia, H. arcticus, Macrothri cidae,
Bosminidae, insec t larvae and notonects.
28
o
o
Ait
o
o
o
Temp
Tundra
°0
.~I:,:m;~~ . :,;,,:;~
°
1
o0
B
o
DOC
:
~.0
0
NOlOneCI
D~p ua Ha r{!o
'
.
°Uir . e
.:He~erOd
0
8
\
Canif
o
Rock
o
rI
-1.0
Figure 2
1.0
RDA 1
Triplot resu lting from
RDA
with species as
response vari ables and
environmental factors as explanatory variab les. The categorical variables desc ribing the type
of pond are added as passive variab les . Samples are represented by circles (roc k ponds),
diamond-shaped (tundra ponds) and square (theIIDokarsts). Polyphe: polyphemus pedicullus;
Holoped: Holopedium gibberum; Chaobor: Chaoborus; Cope.spp: Copepods spp.; Clad.spp:
Cladocerans spp.; Cyclop : Cyc lopoids spp.; Scaphol. : Scapholeberis spp.; Ostracod:
Ostracods; C hydori. : Chydoridae spp.; Notonect: Notonecta; Harpact.: Harpacticoids spp.;
Hesperod :
Hesperodiaptomus
arcticus;
Macrothrycidae spp. ; Calanoi d: Calanoids spp.
Bos mini .:
Bosminidae
spp.;
Macroth .:
29
Daphnia puLex species and clones-environment reLationship
Twenty-three of the ponds surveyed in the pre vio us analysis were assayed for th e
spec ies and gene ti c di versity of th e D. puLex compl ex. A total of twenty- three clones were
di scrim ate d in the overall ponds studi ed from the mi crosate llite analys is. E ight c lones
be lo nged to the D . p ulex spec ies w hil e two c lones be longed to W es tern nea rcti c D . pulicaria
and se ven to Eastern nearc tic D. pulicaria (sensu Colbo urne et al 1998) . One c lone could no t
be di scrimin ated between western and eastern D. pulica ria and four c lones co uld not be
identi fie d to the spec ies leve l. Differences in di stributi on of Daphnia species were obser ved
(Fi g 3). D . puLex c learl y domin ated in roc k bluff ponds w hereas eas tern D . pulicaria
d omin ated in tundra ponds and in therm okarsts. Ei ght cl ones were tripl oids and fifteen c lo nes
were dipl oids. C lo naI diversity averaged 1.64 (± 0.5) c lones per rock ponds, 2 (± 1) c lones pe r
therm o karsts and 2 (± 1) c lones per tundra ponds and l. 88 (± 0 .8) c lones per pond in the
overa ll survey (T a ble 3). No ne of the c lones were present in ail three types of pond s. C lone
Dl was the m os t widespread c lone occ ulTin g in 30 % of the ponds and represe nting 20 % of
the tota l indi vidu a ls of the survey. It was found mainl y on one rock bluff and in one
th erm o karst and was the domin ant clone of 57 % of the pond it inhabited. Ei ght c lones were
re la ti ve ly comm on (D3, T4, D6, D7, DIO, TlI , Dl9 and T21 ). D 3 co-oculTed with o ther
c lones in four ponds but had a lo w abundance compared to D7 which was present in three
ponds but do minated in these ponds . Fo urteen clones were found in a sin gle pond, of them six
compri sed less than 5 % of th e tota l indi viduals gen otyped in the pond and were then
considered as rare c lones. In o ur study Simpson 's index varied from 0 to 0. 5 (T a ble 3). In
genera l, one or few clones domin ated the clonai community. Only five ponds (1 4, l7 , 19, 24
and 39 ) had an index 2': 0.4, indicatin g a simil ar abundan ce of indi vidu als am ong the differe nt
clones.
30
Table 2
Description of clones, percentage of ponds in which eac h c lone was detected,
perce ntage of individuals belonging to each clone in ail the survey and events of co-occurence
with name of clones in the sa me pond (name of c lone: D
= diploid, T = trip loid, the number
are given in order of appearance).
Clone
Species
%
Occure nce
%
Abundance
Coexistence
Dl
D2
D3
T4
D5
T6
D7
T8
D9
DIO
Tl1
DI2
D13
D14
T15
D16
T17
D18
D19
D20
T21
T22
D23
pulex
pulex
pulex
wes rern pulicaria
n. 1.
pulex
pulex
pulex
eastern. pulicaria
easte rn pulicaria
eas tern pulicaria
jJulicaria
pu/ex
eastern pulicaria
n. 1.
n. 1.
n. J.
pu/ex
eas te rn. pu/icaria
western. pulica ria
easte rn. pulicaria
n. 1.
eastern pulicaria
30
4
17
17
4
9
12
4
4
9
9
4
4
4
4
4
4
4
9
4
9
4
4
20
2
5
9
1
8
13
4
D7 , D3, D3 , D3 , D2, D18
Dl
Dl , Dl, D13 , Dl
D5 , T6
T4
T4
Dl, T4
...,
:>
2
7
4
...,
:>
4
0
0
0
0
7
0
5
1
0
DIO
D9 , D19
T21 , T22
D3
T15 , D16, Tl7
D14, D16, T17
D14, T15 , Tl7
D14, T15 , D16
Dl
D IO, D20, D23
D19 , D23
TIl , T22
Tll , T21
D20, D19
31
100% ~----F=====~-----------r------'-----------
o n.i .
• puli
o epuli
50%
~wpuli
o pulex
0%
+-----~----~----~----~~----~----r_--~~----~----~
rock
Figure 3
karst
Relative abundance of Daphnia species
tundra
111
the three types of ponds. Pu li :
Daphnia pulicaria; epu li: Eastern Daphnia pulicaria; wp ul i: western Daphnia pulicaria;
pulex: Daphnia pulex
32
Table 3
Type of ponds , number of clones detected in each pond, ploidy levels and
Simpson's index of diversity (l-D) on clones.
No. of
pond
1
2
3
8
Rock bluff
10
11
13
14
15
l7
18
19
Thermokarst 24
25
26
27
28
29
Tundra
33
34
39
40
41
Min .
Max.
Mean
Clone
richness
1
2
2
1
2
2
2
2
1
2
1
2
3
1
2
3
1
1
2
4
2
2
1
4
1.88
Ploidy
levels
2n
2 n, 3 n
2n
3n
2n
2n
2n
2n
3n
2n
2n
2n
3n
3n
2 n, 3 n
2n
3n
3n
3n
2 n, 3 n
2n
2n
2n
I-D
0.0
0 .3
0.3
0.0
0.3
0.3
0.1
0.4
0.0
0.5
0.0
0.5
0.5
0.0
0.1
0.2
0.0
0.0
0.3
0.3
0.4
0.3
0.0
0.0
0.5
0.2
The infl uence of 14 environmental variab les on the distribution of clones was assessed
using RDA (Fig 4). The first two RDA axes, with eigenvalues of 0.160 and 0 .073 for axis 1
and 2, respectively, accounted for 23.3 % of the variation in species data. RDA axes 1 and 2
explained a high proportion (81.1 %) of the variation in the species-environm ent relationship.
The species-environment correlations for axis 1 (0.89) and axis 2 (0.87) were high, indicating
a reasonably strong relationship between the clones and the environmental variables. With
33
fOl'wa rd selection and Monte Carl o permutati on tes ts RDA ide ntified three vari abl es th at
expl ained significant (p :S 0.05 ) proporti on of the vari ati on in the clones data. T hese were
conductivity, pH and de pth , whi c h accounted for 38.8 % of the vari ation in c lone distribution.
Low conductivi ty seemed to positive ly influence the di stribution of tripl oid c lones (a il species
inc luded) and was cOITe lated with tundra ponds whereas hi gh conducti vity, fo und notabl y in
roc k ponds, near the Hudson Bay, favoured the occ ulTence of dipl oid c lones of Daphnia
pulex. Low pH de termined the occ ulTence of eight c lones (Tl l , D 12, T22, T IS, T 17, T2 l ,
D1 4, D1 6) a il belo ngi ng to eas tern D. p ulicaria and non identi fied Daphnia. One can suppose
th at, accordin g to the tripl ot and the sm ail angles between their aITOWS, the non identified
Daphnia be long to the eas tern D. p ulica ria spec ies. Hi gh pH, fea tu re of som e roc k ponds,
de termined the occ urrence of DS and T4 and in general, of wes tern D. pulicaria. Six clones,
ail of eastern D. Pulica ria (DI O, D9, D2 1, D23, Dl 9 and D7) were clustered together and
were correlated with depth. They occ urred mainl y in tundra ponds.
34
o
o
N
-<
Cl
~
Cond
<>
<>
D5 :
wpuli
0
0
: pH
T"""
1
-0 .9
Fi gure 4
1.1
RDA 1
Tripl ot res ultin g from
RDA
with
clones
as
response
varia bles
and
environm enta l fac tors as ex planatory vari abl es. The ca tegori cal vari a bles desc ribin g the type
of pond and the species of Daphnia p uLex comp lex are added as pass ive vari ables . Samples
are represented by circles (roc k ponds), diam ond-s haped (tundra ponds) and square
(th erm okarsts). Puli: Daphnia p ulicaria ; epuli : Eastern Daphnia pulicaria ; wpuli : wes tern
Daphnia p ulicaria; pulex: Daphnia puLex
35
1.5
Discussion and conclusion
A growing interest has been devoted toward research on fres hwater ecosys tems at hi gh
latitude. Main limnological data on arcti c a nd subarctic lakes and ponds have been recorded
from A laska to Pinland (Rauti o 1998; Swadlin g, Pi enitz et al. 2000; Swadlin g, Gibson et al.
2001 ; Ra utio and V incent 2006). Heterogeneity of the fores t-tundra region geomorpho logy in
Subarc ti c Québec res ults in great vari ati on of environmental condi tions in shall ow
waterbodi es. Thi s represents a good opportunity to study fundamental species-environment
re lati onships. Thi s study aimed to charac te ri ze Subarctic Québec shall ow ponds and examine
the di stributi on pattern and abundance of zooplankton and its re lati onship with abi oti c and
bioti c gradi ents under three sca les of vari ati on, community, in terspec ific and intraspecific.
In thi s study, th e pH ranged from 6.0 to 8.7. Rock bluff ponds experi enced the widest
pH range as both minimum a nd max imum va lues of the survey have been enco untered there
but had a mean pH va lue hi gher than that of tundra or therm okarst pond . Thi s can possibl y
res ult fro m di ssoluti on of the carbonate bedrock th at formed the bas ins, as proposed by
Swadlin g and coll. (2001 ). In peatl and ponds, it has been show n th at hi gh concentrati on of
humi c matter can be responsible for the more ac idic waterbodies (ArIen-Pouli ot and Bhiry
2005 ). Very high values of conductivity of some roc k ponds are due to the salts cani ed in
seaspray from nearb y Hudson Bay where conducti vity has been recorded as 38.3 mS cm-
I
durin g the samplin g peri od. Concentrati on of DOC in thi s study reac hed ex tremely hi gh
values in sorne ponds. Except for pond 13 whi ch we menti oned above, the hi gh va lues were
mainly fo und in therm okarsts (mean 2 1.7 m g L· I ) with hi gher va lues in pond 19 (35.7 m g L· I )
and 20 (37. 5 mg L· I ). These ponds are located at the base of a palsa and are comple tely
sunounded by vegetation (mostl y Sphagnum, pruces and m osses). Thus, input of DOC from
36
telTes tri al sUlToundin g vegetation mu st be relati vely hi gh. Moreover, the deep tea color of the
water indicated that there was a lot of humic matter in suspension in the water column , also
contributing to hi gh va lues observed of di ssolved organic carbon. DOC is an important abi otic
factor in ail aquatic ecosystems . It is a potential energy source for zooplankton (Sa lone n and
Hammer 1986; Ari en-Pouliot and Bhiry 2005 ) and can be a protection against li ght
penetration . It partic ipates to the absorpti on of ultraviolet radiati on through the water column
and therefore helps protecting aquatic organi sms from the damaging effec t of UV
wavelengths (Davis-Colley and Vant 1987) . At hi gh concentrations however, it can partly
inhibit photos ynthesis and primary production as it limits li ght penetration. Surpri singly,
chl orophyll a concentrations reac hed the ir maxima values (15 .80 Jlg C l) in those two
thermokarsts (ponds 19 and 20). Those hi gh va lues may be biased toward the input of tree
leaves and peat-moss degradati on in the ponds. In general , the chl oroph yll a co ncentrati ons
fo und in this study are comparable to those reported in a previous study in the same region
(Swadling, Gibson et al. 2001). Zooplankton abundance also vari ed greatly (between 570 and
1 427 000 ind m-3 , in ponds 36 and 13 respecti ve ly); the maximum abundance was found in
pond 13 that reac hed the max imum concentration of DOC and was constituted mainly of
Daphnia and chydoridae. This high va lue may be due to an aggregation of individuals in the
amount of water sampled. In fact, we often observed crowded 'clouds' of Daphnia. Detailed
examinati on of zooplankton showed that Daphnia and copepods together dominated the
community in the three quarte rs of the ponds sampl ed. Inside the copepod taxa, we found th at
Leptodiaptomus minulus was the dominant copepod species, following by Hesp erodiaptomus
arcticLls and Microcyclop rubellus .
Zooplankton- environment relationship
37
The RDA analysis revealed that temperature, conductivity, altitude and DOC togethe r
explained 25 .5 % of the variation in species composition . No othe r variabl e te sted was
stati sticall y significant. The biotic factors tes ted here (c hl ol'Oph yll a, phaeopi gment) did not
inf luence zooplankton assemblages. If the concentration of chlorophyll a can be a good
es tim ator of phytoplanktonic biomass, it does not give informati on on food quality. The food
particle size preference can be different among filter-feeding zoopl a nkton species. For
ex ample, cladocerans of arc tic and subarctic ponds grazed most efficientl y on bac teri a and
picophytoplankton whereas copepods tended to feed on larger food particules (Rauti o and
Vi ncent 2006). Examination of the ph ytoplankton of few ponds in this study revea led that
nanopl a nkton via small flagellates (:s 10 !lm) dominated the total bi omass (data not show n).
Moreover, it has been demonstrated that benthic algae represent a high proportion of food
availab le for grazing taxa (Rauti o and Vincent 2006; Rauti o and Vincent 2007). Small value
of pe rcentage of vari ance explained by envil'Onmental variables is typical to species data and
is due to large number of taxa and many zero values (TerBraa k 1994). Moreover, other local
or regiona l factors may play a signi ficant l'Ole in species di stributi on. Testing for the effect of
competition or predati on in a whole zoopl ankton data set is a difficult task because eac h
in vertebrate predator as we Il as eac h potential competitor is a part of the zooplankton
community. Excluding those species from the species matrix and including them in the
ex pl anatory variabl es matrix changed the overall analyses. H owever, durin g the data
screenin g, even with this data set structure, we found no evidence of an affec t of predation on
the remaining species.
Temperature was the mos t important factor structurin g zooplankton communities in the
ponds surveyed. Water temperature is mostl y a function of air tempe rature and volume of
ponds. Larger and deeper ponds buffer temperature variation m ore efficientl y than small
38
shallow ponds. As a res ult, the di stributi on of zooplankton taxa in habitats of different size
can be dependant on the temperature tolerance range of species, range that can vary broadly
am ong species, even c losely-related. Daphnia are th ought to be favoured by low temperatures
(S teiner 2004), whil e increasing temperatures may favo ur small-bodi ed cladocerans
(Bengtsson 1987). T hi s was also seen in thi s study as Chydoridea were strongly assoc iated
wi th hi gh temperature whereas Daphnia had medium temperature preference. Calanoid
copepods and Polyphemus pedicullus prefelTed low tempera tures . However, one must be
careful with the effec t of thi s parameter on zoopl ankton communities . Here, we evaluated a
single meas urement of temperature on species di stributi on. As it is hi ghly cOlTelated with air
temperature, a several degree difference between days would mos t like ly influence the res ults.
Harpacticoids, insec t larvae (except Chaoborus), notonects, Dap hnia and Hespe rodiaptomus
arcticus were assoc iated with increased conducti viti es. Conducti vity has already been show n
to be an important fac tor contributing to copepods (Swadling, Gibson et al. 2001 ) and
Daphnia pulex (Wei der 1987) di stribu tion in thi s region.
Studi es on the species di stributi on of zoopl ankton along an altitudinal gradi ent have
demonstrated th at species ri c hness dec reased with increasing altitude (Patalas 1964; Hebert
and Hann 1986; Rauti o 1998), which is in accordance with the predi cti on of island
biogeograph y (M acArthur and Wil son 1967) that the more isola te the habitat is, the less it
would have spec ies able to coloni ze it. However, the altitude gradient in this study is
relative ly low (from sea-level to 2 11 m hi gh) and hi gh altitude ponds were not assoc iated with
less taxa here. Nevertheless, we have to keep in mind th at species ric hness among certain taxa
as copepods, c hydoridae, macrothi cidea and bosminidae were not eva luated here . The
signifi cant effect of altitude on species di stributi on may probably be a result of interac ti ons
with other factors (peatl and catc hment, vegetati on type and water c hemi stry, for exampl e)
39
rather th an colonisation ab ilities of zoop lankton taxa. Neverthe less, altitude gradien t is a good
discriminator of our types of ponds in this region; the rock bluff ponds being near the sealevel, tundra ponds between 30 to 100 m hi gh and th en therm okarts up to 210 m hi gh. Finally,
DOC was the fourt h factor contributing signifi cantl y to the zoopl ankton community structure.
Low DOC concentrations were tolerated by onl y few species, Holopedium gibbe rum,
Polyphemus pedicullus and calanoid species. Except if they require low DOC concentrations
as Holopedium gibberum (Leec h, Pade le tti et al. 2005), low protec tion from UV radiations
offered by DOC in tundra ponds may be compensated by vegetation catchment of those ponds
that can act as a refuge for the se species.
Daphnia pulex species and clones-environment relationship
C lonai di versity of Daphnia pulex in the studi ed region was 1.86 c lones per pond.
Previous investigations of clonai diversity of the D. pulex complex on the other side of the
Hudson Bay in the C hurchill region (Manitoba) counted an average of 2.9 clones of D.
tenebrosa (Dufresne and Hebert 1995), a species not found eastern of the bay and a verages of
1.5 and 1.7 clones per ponds for pi gmented and non-pi gmented D. pulex, respectively
(Wei der and Hebert 1987). Southerly, Hebert et al. (1988) fo und a c lonai di versity of
obligately parthenogenetic populations of D. puLex of 2.8 clones per pond in the Great Lakes
region . Our res ults are th en in agreement with th e general ass umption that species (here,
clonai) di versity decreases with increasing latitude.
In general, ponds were domin ated by one or few clones. Local adaptation and local
genetic differe ntiation may explain the ecological di vergence between clones. Hebert et al .
(1988) qualified clones as ecotypes because they di ffered physiologically and had different
40
ecological demands. The RDA showed interesting patte rn in clonai assembl ages. Of ail the
vari ables considered here, conducti vity, pH and depth accounted for a grea t perce ntage of the
vari ation in c lones distributi on. Triploid clones are res tri cted to low conduc ti vi ty water bodi es
whil e dipl oid cl ones of D. pulex was found m ostl y in rock ponds that ex hibited relative ly hi gh
conductivities. These results contras t with previous works on D. pulex clones by W e ider and
He bert (19 87) and W ilson and He bert (1992). In C hurchill ponds, melani c clones, th at are
also polyploids, are found in roc k bluff ponds and at much hi gher salini ties th an polyp loids of
the Kuujjuarapik region and dipl oid are res tri cted to ponds with hi gh humi c content. It seems
that polypl oids are not more to lerant to a broader range of ecologica l conditi ons but the ir
apparent segregati on may in vo lve compe titive abilities toward dipl oid clones. Thi s
competiti on can in volve some of their life-history c harac teri sti cs . For ex ample, polyploid
Daphnia are generall y lat'ger th an their dipl oid counte rpart. As in verte brate predator such as
the larvae of the phan tom midge Chaoborus prey on small Daphnia, thi s size-dependant
in vertebrate predati on could parti y expl ain the di stributi on of di fferent cytotypes of Daphnia,
th ough nothing in thi s study supports this as sumption . Dufresne and Hebert (1997)
de monstrated that in North A meri ca, D. pulicaria is divided into three di stinct Iineages th at
ori ginated from different gl acial refu gia. W e have seen that in thi s study region, Nunavut, two
of th ose Iineages are sympatri c . However, despite the ir close taxonomie and ph ylogeneti c
relati onship, wes tern and eastern pulicaria species had different pH tol erance (see Fi g. 4) that
permits them to inhabit different habitats thus escaping from intercompetiti on. A similar
pattern exists within D. p ulicaria and D. pulex. In eastern Canada, it is admitted that Daphnia
p ulicaria is restricted in lakes w hereas D. pulex is fo und in ponds (Cerny and Hebert 1993).
However, in thi s study, D. pulicaria was fo und in more th an ha If of the ponds studi ed. In
oppositi on to th e hypothesis th at it occ urs in large surface are a water bodi es (ie . lakes)
because of hi gher environmental heterogeneity, here, we argue that depth is a de terrninant
41
factor in species di stributi on of the D. pulex compl ex. RDA shows that the OCC UlTe nce and
ab undance of D. pulicaria is positively con elated with depth and that D. p ulex is fo und
mainl y in roc k ponds that are generall y less deep than tundra ponds or therm okarsts.
T his study provides a new integrati ve approach in fundamental ecology because it gives
an in sight of the re lationship between different sc ales of biodi versity and its environment.
Bioti c and abioti c factors were expl ored to exp lain structure of community of th e who le
zoopl ankton, then we foc used on one of its component, the Daphnia p ulex compl ex and
searched for ecological pattern s among species and inside species, at the gene ti c level. At the
three scales, ph ysicoc hemi cal fac tors expl ained directly the di stributi on of organi sm s. If one
environmenta l conditi on favo urs the occ urrence of one species, it is difficul t to attes t for a
better fitness unde r thi s conditi on or a better competiti ve ability or for the possibility th at thi s
species was the first coloni zer of the pond and tha t no sub sequent species had di splaced it.
In teractions betwee n abi oti c, bioti c and hi storical fac tors can be ana lysed in experimental
studi es. Wil son and Hebert (1 992) es tablished th e selec tive importance of abioti c factors,
competi tion and preda tion in structurin g clona I assemb lages of D. p ulex in a study combinin g
observati ons of di stributional pattern s with experimental manipul ati ons. In situ observations
showed that physical factors such as salinity contributed to the spati a l repartition of clones. In
experiments, they highli ghted the role of competition in limiting clonaI ri chness in ponds and
also fo und an interac ti on between compe titi on and predati on on the ability of certain clones to
coloni ze and domin ate habitats in their ran ge of ecol ogical tolerance.
The wide ecological tolerance assoc iated with zoopl ankton of subarctic ponds and the
more or less large overl ap in taxa di stribution may be expl ained by hi gh species and gene tic
di versity. In thi s study, the genu s Daphnia was absent from onl y six of the forty-three ponds
42
surveyed and di stributed in ail types of ponds over broad environm enta l conditi ons. However,
interspecific di stribu tion of Daphnia vari ed markedl y. Spec ies di stributi on was cOlTelated
with environmenta l gradi ents. The great vari ati on in clonai di stributi on indicated that there
was a strong genotype-environment interac ti on as proposed by W eider (1 987). Pl oidy level
had certainl y an effect on the di stribution of clones, as dipl oids and polyploids rarely
coexisted in ponds and triploids were segregated in low conduc ti vities water.
From thi s study, we can hypothesize th at the more ge netic di vers ity a species has, the
widest di stributi on and ecological tolerance it will have. In order to predi ct bi odi versity
c hanges res ultin g from climate warming in subarctic and arc ti c regions, it is thu s necessary to
have an insight on th e ge neti c di versity of species and their interacti on with the surroundin g
ecosys tem . Further studies are needed on the other zooplankton taxa in order to have a better
understandin g of bi odi versity in fres hw ater ecosystem s from hi gh latitudes.
43
57
and Hebert (1987) also revea led ecop hysiological differences among D. pulex clones from tbe
Cburc bill region (Manitoba) and suggested tbat tbose clones were not ecological equivalents .
Our results suggest no difference in aerobic and anaerobic capacity (estimated by ETS,
CS and LDH enzyme capacities) among D. pulex clones after short-term acclimatization to
stressful conditions as we Il as no ability to adju st metabolic capacities to these conditions.
Studies that have observed different ETS activities among different crustaceans species had
linked tbis variabi li ty to different environmen tal conditions tolerance and spatial distribution
(S imcic and Brancelj 1997; Yurista 1999; Simcic 2004; Simcic 2006). In contrast to these
studies wbicb showed differences at interspecific level, the present work did not show any
metabolic differences at the intraspecific leve l. Since body mass can affect metabolic rate and
enzymatic capacity, mass must be considered when comparing metabolism capacities to avoid
bias due to allometric factor. Here, apparent e nzymatic differences were mostly explained by
mass variabi lity. Since enzyme capacities are not influenced by pH or temperature and did not
differ among clones, it seems that differential distribution of D . puLex clones used in this work
is poorly influenced by metabolic capacity. Our results stand in contrast to res ults from
previous studies that noted a relationship between metabolic differences and species
distribution (Simcic and Brancelj 1997; Simcic 2004; Simcic 2005; Simcic 2006). Since mass
highly influences metabolic capacity it would be of great interest to revisit their conclusions
considering the possible mass differences among species .
Briefly, metabolic capacity of D. pulex clones used in this study did not vary among pH
and temperature treatments . Moreover, aerobic and anaerobic capacity did not differ among
clones. In the light of our results, metabolism does not seem to exp lain geographic repartition
pattern of D. puLex clones used. Other parame ters such as body mass and life-history traits
58
coul d explain the di stributi on of polypl oidy D. puLex cl ones. A previous study on D. pL/Lex
li fe- hi story traits has shown large inter-c lonai vari ability in response to temperature shi fts
within eac h ploidy level (Dufresne and He bert 1998). Thu s, to confinn or infirm the influence
of metabo li sm on the di stributi on of D. puLex clones, future researc h shou ld examine more
c lones of each ploidy level and explore different enzymes affec ting the di fferent e nergetic
pathways.
2.6
Acknowledgments
Thi s work was supported by a researc h grant From the
atural Scie nces and E ngineering
Research Countil (NSERC) of Canada and by a Canadi an Funds for Inn ovation (CFI) to
France Dufresne. W e thank Pien e Bli er for insightful comments on th e manu sc ript. Caro line
Jose acknow ledges a sc holarship form BioNord From the Uni versité du Qué bec à Rim ouski.
Etienne Hébert C hatelain was fin anciall y supported by Fondati on de l'U ni versité du Québec à
Rim ouski .
S9
60
CHAPITRE 3
Differentiai survival among genotypes of Daphnia pu/ex differing in reproduction
mode, ploidy level and geographic origin
Caroline Jose and France Dufresne *
Article submitted in the j ournal Evolutionary Ecology
Laboratoire de Bi ologie Évolutive, Département de Biologie, U ni versité du Québec à
Rimouski,
300 A llée des Ursulines, Rimou ski , Qué bec, Canada GSL 3A 1
* COlTesponding author. Tel. : 14187231986 #1438; fax: 1 4187241849.
E-mail address :france-dufresne@ ug ar.gc.ca
61
3.1
Abstract
The di stributi onal pattern of geographical parth enogenes is has not been c1earl y
eXplained yet. In Daphnia pulex, asexuals are found at hi gher latitude and in more marginal
habitats than their sex ual relati ves. Sorne asex ual lineages have also becom e polyploid. In
order to understand thi s spati al di sjunct di stributi on among different cytotypes of Daphnia
pulex, thi s study aimed to test if polypl oid clones were more res istant th an sympatri c dipl oid
clones to a wide range of environmental fac tors and if asexual Daphnia (diploid c lones) are
more tolerant of ex treme environmental conditions th an sex ual ones. W e report signi ficant
differences in survivorships after short-term exposure to acute pH, conducti viti es, and
temperatures in 12 clones of the Daphnia pulex complex. However, ploidy level and
reproduc ti ve mode did not influe nce survivorship . Historical fac tors could pl ay a substanti al
role in generatin g the pattern of geographical parthenogenesis. L ocal adaptati on and the
incomple te isolati on of reproductive modes are indeed very important in expl ainin g the
ecological di fferences among clones.
Keywords: Daphnia pulex, geographica l parthenogenesis, polypl oidy, to lerance,
temperature, conductivity, pH
62
3.2
Introduction
Asex ual organi sms frequently dominate at hi gher latitudes and altitudes and in more
marginal habitats than their sex ual relatives, a pattern refened to as geographi cal
parthenogenesis (VandeI1928). Even thou gh thi s di sjunct spatial di stributi on has been known
for almos t a century, we still lack an adequate explanation for it. Different types of non
mutually-exclusive hypotheses (hi storic al, demographic, ecologica l, and evoluti onary) have
been proposed to explain geographical parthenogenesis . Dem ographie hypotheses refer to the
fact that asex uals do not pay the twofold costs of sex (male producti on) and therefore have a
numerical advantage over sex uals (Maynard-Smith 1998). Asexuals ca n establi sh pop ul ations
from a sin gle individual and hence can colonize remote areas more rapidly than sexuals
(reproductive assurance hypothesis,(Baker 1955; Cue l1 ar 1994)). They do not suffer from
ge ne tic bottlenecks at low popul ati on densities and hence may outcompete sex uals at the edge
of a geographic al range (Peck, Yearsley et al. 1998 ; Parker and N ikl asson 2000; Haag and
Ebert 2004). Under an ecological scenari o, asex uals are better competitors or have fitness
advantages over sex uals under certain ecological conditions (Glesener and Tilman 1978),
owing large ly to their frequent hybrid origins. Hi storical explanati ons refer to the association
between parthenogenesis and environments that were strongl y affected by the Pleistocene
glacial cycles (S tebbins 1984 ; Kearney 2005). The repeated advances and retreats of glaciers
has resulted in the creati on of refugial races which were free to colonize new environments as
glac iers retreated. Due to thei r better colonizing abilities, asexuals may have been ab le to
fo ll ow glac ial retreats faster th an sex uals, hence the pattern of geograp hi cal parthenogenesis.
One important confou ndin g fac tor of asexuality and geographi cal di stributi on is the
frequent association of apomi xis and po lyploidy. Ali apomictic angiosperms are polyploid
63
(As ker and Jerlin g 1992) but not aIl polyploid plants are asexual. In the Anim al Kin gdom, the
associati on between polyploidy and parthenogenesis is present in two thirds of taxa, m os tl y
insec ts and reptiles (S uomalainen, Saura et al. 1987; Otto and Whitton 2000).
«
Geographical
polyploidy » has been coined by zoologists to emph asize the ra ie played by polypl oidy in
geographi cal parthe nogenesis (Littl e, DeMelo et al. 1997; Stenberg, Lundm ark et a l. 2003).
Genome polyploidi zati on may res ult in unique gene combinati ons, an altered ex pression
pattern s, enl arged body, or cell size and elevated level of he terozygosity (Parker and
Nikl asson 2000). Thi s has been argued to make po lypl oid clones more tolerant to abi otic
stress and to possess wider ecological tolerances than their sex ual ancestors (Lewis 1980) i.e.
general-purpose genotype (Lync h 1984). Few studi ed have been conducted to examine
ecological and ph ys iological tolerances of natural asexuals/polypl oids taxa with re lated
sex uals.
Daphnia pulex, a cosmopolite freshwater microcrustacean, has a wide di stributi on in
North Ameri ca, bein g found from Mexico to the Arctic (Hrbace k 1987). Temperate
popul ati ons of thi s spec ies reproduce by cyclical parthenogenesis, th at is an alternati on
between asexu al (through the production of unfertilised subitaneous eggs in th e summer) and
sex ual reproducti on (through the production of sexual restin g eggs in th e fall ). Some lineages
have made permane nt transitions to obliga te parthenogenesis owing to genes that suppress
meiosis in females but not in males(Innes and Hebert 1988) . As a res ult males calTying these
genes can mate with sex ual females and most of the res ultin g offspring will reproduce by
obligate parthenogenesis (Innes and Hebert 1988). E astern popul ati ons of Daphnia pul ex are
stri ctly asexual s, mi xed populations (both sexuals and asexuals) are found in On tario whereas
wes tern populati ons reproduce by c yclic parthenogenesis. In additi on, sorne asexual lineages
have also become polypl oid (i.e. more th an two sets of chrosomosomes)(Beaton and Hebert
64
1988). These polyploid lineages are prevalent in arctic and some hi gh alpine areas whereas
diploid lineages dominate in temperate zones (Hebert, Schwartz et al. 1993; Weider, Hobaek
et al. 1999 ; Hebert and Finston 2001; Adamowic z, Gregory et al. 2002; Weider and Hobaek
2003; Aguil era 2007). Ecological differentiations of these clonaI lineages are not we il
understood. Subsistence of clonaI diversity in environmental heteroge neous habitats suggests
that clones have diverged physiologically into ecotypes, res ult of adapti ve diffe renti ation to
diverse environmental conditions (Weider and Hebert 1987; Boersma 1999).
This study aimed to test if 1) polyploid clones are more resistant than sympatric diploid
c lones to a wide range of environmental factors and if 2) asex ual Daphnia (dipl oid clones) are
more tolerant of ex treme environmental conditions than sex ual Daphnia. We report signi ficant
differences in survivorships after short-term ex posure to ac ute pH, conductivities, and
temperatures in 12 clones of the Daphnia pu/ex complex.
3.3
Materials and methods
3.3.1
Origins of clones
Ploidy level, ori gin , reproduction mode and heterozygoty of the twelve clones are
described in table 1. Heterozygoty of the clones was obtained by genetic typi ng usin g seven to
ni ne microsatellite markers (Dpu 12.2, Dpu 12.1, Dpu 40, Dpu 6 Dpu 30, Dpu 45, Dpu 502
and Dpu 183).
3.3.2
Culture conditions
65
Ten adult females of eac h c lone were kept in culture in environm ental grow th c hambers
(Therm oForm a Diurnal Grow th Chamber) (pH 7, 20 oC and 16 h li ght: 8 h dark diurnal
cyc le). Clones were cul tured in 4L aqu ari a with filtered pond water. They were fe d with an
alga l suspension of Selenastrwn sp. twice a week (approx. concentrati on of 100 000 cell slL) .
Alga l conce ntrati ons were standardi zed with a hematocy tometer. The c ulture medium was
c hanged weekl y. C lonai lineages were raised durin g three generati ons, wi th ail mothers sorted
out at eac h generati on to avoid maternaI effec t (Lynch and Enni s 1983).
3.3.3
Tolerance experiments
Ac ute tolerance ex periments were performed fo llowing the USEPA method for daphnid
ac ute tox icity test (USEPA 1982). Pre liminary ex periments were conducted to determine
va lues of conduc ti vi ti es, pH and temperature that would res ult in gradu aI mortali ties over a 48
ho ur period.
Ac ute conducti vities (80 and 1500
~S/c m ),
pHs (5 and 10) and temperature
(30°C) were determined to be in the Daph.nia tolera nce interva l ranges. PHs were adjusted
with drops of NaOH or HCI and conductiviti es with di still ed water and instant ocean salts.
Ovigerous fema le were isolated in 40ml plastic cups. Thirty neonates
«
24h) were randoml y
removed, rin sed with pond water containing no algae, and transfen ed to eac h experimenta l
solu tions. Te n neonates were set-up pe r 40 ml plasti c c ups in tripli cates. Ten additi onal
neonates were transferred into a control solution (pond water, 20 °C, pH 7 and 270 ~S.cm- l ).
Organi sm s were not fe d durin g the experiments and dead ones were counted and removed
after 12h, 24h, 36h and 48 h. Death was assumed when organi sms were immobile for 15s after
they were gentl y shaked. PH tes t soluti ons were renewed dail y to avoid buffer effect and
changes in pH. E xcept for the tempera ture, ail experiments were perform ed in th e 20°C
incubati on chamber. The 30°C test was performed in an incubati on cham ber with a natural
66
diurnallight regime provided by external windows . For ail tests, mortality in controls was less
than 10%.
3.3.4
Statistical analysis
SAS software (9.1.3) was used for ail statistical analyses. KJuskall-Wallis analyses were
performed to test the effect of ploidy level, reproductive mode, geographic origin , clone, and
heterozygoty on survivorship of ail twelve clones (%) at each treatment. Post hoc comparison
tests (non parametric Tuckey test) were used to detelmine wh ich means differed significantl y.
Significance was assessed at the 0.05 (or lower) level for ail tests .
Median lethal time (LT
50)
were calculated using PROBIT procedure in SAS . When
possible, 95 % fiducial limits were used to compare values of LT
3.4
Results
3.4. 1
Survival
50
among clones.
There was no significant effect of ploidy level, geographic origin, and heterozygosity on
survival rates after a 48 h exposure at each treatment (Tab le 2) . Reproductive mode had a
significant effect on the survival of D. pulex clones with sexual clones suffering more
morta lity at 1500j.LS .cm-l and less at pH 5. Clone had a significant effect on survivorship at
ail treatments (table 2) . Non pm'ametric multiple comparisons Tukey test (p
:s: 0.05) revealed
differences between DAT-3 and DST-8 clones at 30°C. Also, there was important intrac lonal
variability according to their standard deviation (Figure l, 2 and 3).
67
3.4.2
Median lethal time
Median lethal time (LT
50)
differed among clones at each treatment and fiducia l limits
often overlapped (table 4). Some clones showed no or not enough variations in mortality so it
was not possible to build a regression slope. Consequently, those clones, without 95 % fiducial
limits values, could not be considered.
Acute conductivities. At 80 uS.cm -1 PN-232 and DST-102 suffered no mortality. No
differences in tol erance to low conductivity were observed among PN-9 , DATAI and DAT-3
and between DAN-86 and DAN-52. Also these last two died sooner than DATA!. PN-9 was
more tolerant than DAN-52 and DAT-3 had simi lar mortality than DAN-86 . At 1500 uS .cm-
1
no mortality was noted for DAT-3 , DAT-lO, DAN-52 and PN-232. DA -86 and PN-9 had
greater high conductivity tolerance than DST-l.
Acute pH. DST-l , DST-102, DATAI and DAT-3 suffered no mortality at pH 5. PT-I21 was the less tolerant clone at this treatment; ail others hav ing overlapping fiducial limits.
At pH 10 no mortality was observed for DAN-86. DST-102 was the less tolerant c lone; ail
others showed similar tolerance at acute high pH.
Acute temperature. At 30°C, no mortality was observed for DST-8 . DST-l with PN-95
and DAN-52 had greater tolerance than DST-102, DATAI and PN-232 . PN-232 was less
tolerant than DST-102.
68
Distributions and reproductive characteristics of twelve clones of Daphnia
Table l
puLex .
Ploidy
Reproduc tion
Geographic
level
mode
ong1l1
DST-l
2
Sexual
DST-8
2
DST-102
Cl one
Lati tudelLongi tu de
Heterozygoty
Minnesota, U.S.
45 °11/93°34
0.89
Sexual
Ontario, CA
42°34/80°15
0.33
2
Sexual
Illinois, U.S.
40°07 /88 ° 12
0.33
DATAI
2
Asexual
Quebec, CA
50°09/70° 1
0.53
DAT-3
2
Asexual
Ontario, CA
43 °44/80°57
0.89
DAT-lO
2
Asexual
Michigan, U.S .
42°75/84°54
0 .89
DAN-86
2
Asexua l
Quebec , CA
55 °18/77°55
0 .78
DAN-52
2
Asexual
Quebec, CA
55 ° 18/7r 55
0.86
PN-95
3
Asexual
Quebec, CA
55 °18/77 °55
0.4
PN-232
3
Asexual
Quebec, CA
55 ° 18/77°55
0.67
PN-9
3
Asexual
Quebec, CA
55 °19/77°29
0.73
PT-1-21
'"-'
Asexual
Ontario, CA
42°16/82°58
Table 2
Effect of c lone and reproduction mode on survival rates after a 48 h ex positi on
to each treatment according to Kruskal Wallis test.
d.l.
X2
P
1
8,1763
0,0042
1
7,7379
0,0054
Il
20,6904 0,0367
1500jlS.cm-1
Il
29,0053 0,0023
pH5
Il
21,4459 0,0290
pH 10
11
21,7369 0,0265
30°C
Il
26,1530 0,0062
Treatment
Reproduction
1500jlS .cm-
1
pH5
Clone
°
80jlS.cm
-1
69
140
80 uS.cm- 1
120
100
~
ro
r--
80
-.,
>
'>
....
::l
60
CI"J
40
l'
20
,I,
0
140
1500 uS.cm- 1
120
100
~
ro
80
>
"E
::l
60
CI"J
40
20
0
li
~
CI"J
Cl
L-
00
~
CI"J
Cl
a
C'l
~
C/l
Cl
Figure 1
"<t
~
«
Cl
<')
~
«
Cl
a
-
~
«
Cl
z« z« z
'D
00
C'l
<n
<n
0\
0..
Cl
Cl
C'l
<')
C'l
Z
0..
0;Z
0..
C'l
-
~
0..
Mean (S.D .) survival (%) of twelve Daphnia pulex clones (n
= 3) exposed to a
48 h ac ute conductivity. DST: dipl oid sex ual clones from temperate regions; DAT: dipl oid
asex ual c lones form temperate regions; DAN : diploid asexual from Nordic region; PN:
polyploidy clones from Nordic region ; PT: polyploidy c lones from temperate regions.
70
pHS
140
120
100
80
60
40
,,--...
~
20
'--'
~
>
0:;
.....
140
pH 10
::l
U)
120
100
r-
80
l'
60
1··
40
20
0
'--
0
L-
V)
0"-
Z
c..
Figure 2
Mean (S.D.) survi val (%) of twelve Daphnia pulex clones (n
= 3) exposed to a
48 h acute pH. DST: diploid sexua l clones from temperate regions; DA T: diploid asexual
clones form temperate regions; DAN: dipl oid asex ual from
ordi c region ; P : polyploidy
clones from Nordi c region; PT: polyploidy clones from temperate regions o
71
Figure 3
Mean (S.D.) surviva l (0/0 ) of twelve Daphnia puLex clones (n = 3) exposed to a
48 h ac ute temperature. DST: dipl oid sex ual clones from temperate regions; DAT: diploid
asex ual c lones form tempera te regions; DAN : dipl oid asex ua l fro m Nordi c region; PN :
polypl oidy clones from Nordi c region; PT: polyploidy clones from temperate regions.
*
means statisti cal di fferences between treatment according to mul tipl e compari sons Tu key test
(p ::; 0.05).
72
Tabl e 3
Median lethal time (LT
50)
and 95 % fiduciallimits for eac h clone at 80 and
1500 uS .cm-1. 0 in parenthesis mean that no mortality was observed until 48h. Dashes in
fiducial limits are present when there was just one point to make the regression slope.
95 % Fiducial
limits
95 % Fiducial
limits
9.2
37.2
Clone
LT 50 (h)
80uS.cm- 1
DST-1
13.9
LT 50 (h)
1500uS.cm-1
24.1
DST-8
6.0
30.2
DST-I02
- (0)
- (0)
- (0)
DAT-41
58 .8
49.0
85.8
51.2
51.2
DAT-3
47 .8
4l.6
59.4
- (0)
- (0)
- (0)
DAT-lO
32.3
- (0)
- (0)
- (0)
DAN-86
37.3
33.0
42.9
47.1
42 .9
54.6
DAN-52
25.0
8.8
37.5
- (0)
- (0)
- (0)
PN-95
P -232
52.0
- (0)
- (0)
- (0)
- (0)
- (0)
- (0)
PN-9
46.2
39.8
57.8
51.3
44.5
65.8
PT-l-21
31.4
172.7
52.0
73
Table 4
Median lethal time (LT 50) and 95 % fiduciallimits for each c lone at pH 5 and
pH 10. 0 in parenthesis me an that no mortality was observed until 48h. Dashes in fiducial
limits are present when there was just one point to make the regression slope.
Clone
LT 50 (h)
95 % Fiducial
limits
- (0)
- (0)
LT 50 (h)
pH 10
53.9
83.9
95 % Fiducial
limits
49.0
78.3
59.5
3148 .0
48.5
37 .1
DST-l
DST-8
pH 5
- (0)
93 .9
DST-102
DATAI
- (0)
- (0)
- (0)
- (0)
- (0)
- (0)
4l.7
55 .3
DAT-3
- (0)
- (0)
- (0)
DAT-lO
DAN-86
42 .2
60.4
28.4
416.7
49 .5
92.8
77.8
82.4
- (0)
DAN-52
45.2
PN-95
PN-232
47.5
49.4
39.5
43 .9
55.0
54.0
140.1
72.7
55 .8
337.1
63 .3
52.6
615 .3
PN-9
PT-I-21
51.2
25.5
56.9
94.6
49.9
91.8
22 .2
28.8
46.4
77.0
57 .5
237 .9
632.4
59.1
- (0)
- (0)
74
Table 5
Median leth al time (LT 50) and 95 % fiduciallimits for eac h c lone at 30°e. 0 in
parenthesis mean that no mortality was observed until48h. Dashes in fiduci allimits are
present when there was just one point to make the regression slope.
3.5
95 %
Clone
LT
DST-l
DST-8
80 .5
- (0)
58.5
- (0)
DST-I02
DAT-41
42.8
45.4
23.3
40.4
-7.1
DAT-3
18.9
DAT-lO
48.8
DAN-86
47 .9
42.7
57.8
DAN-52
66 .1
53 .5
234.3
PN-95
52.4
46.2
67.0
PN-232
27.1
23 .5
30.8
PN-9
43.5
38.2
52.0
PT-I-21
49 .9
50
(h)
30°C
Fiducial limits
382.3
- (0)
45.4
Discussion and conclusion
The results of the present study indicate that Daphnia pulex clones vary in their ac ute
tolerance to ail the different conditions met in this experiment. However, there was no
evidence that po lypl oid clones are more tolerant to ex treme conducti viti es, pH and
temperature th an diploid ones. Contrasting results have been obtained in previous studi es on
Artemia species. Zhang and Lefcort (1 991 ) found a positive relationship between polyp loidy
and resistance to environmental stress in A. parthenogenetica. Polypl oids had hi gher survival
rates than sympatric dipl oids after a short-term exposure to cold and heat shocks. Similar
results were obtained by Barata et al. (1996) with tetraploid Artemia strains havin g hi gher
75
survival at 15°C than th e dipl oid ones although they had similar survival at 30°e. In contrast
to the hypothesis that polyploidy confers greater tolerance to ex tre me e nvironm enta l
condi tions because of hi gher heterozygozity and metabolic f1 exibility, Lic ht and Bogart
(1 989) fo und a simil ar therm al tolerance between dipl oid and tripl oid salamanders
(Ambys toma laterale-texanum)
and
Sc hultz
(1982)
demonstrated
th at
tripl oid
fish
(Poeciliopsis monacha-lucida) have lower re sistance to cold stress th an dipl oids. In thi s study,
polyploid and dipl oid clones of Daphnia pulex have a similar tolerance to a range of
environm ental fac tors. Therefore their geographic di stributi on can not be expl ained by an
increased to lerance to more ex treme environmental conditi ons. It has bee n sugges ted that
hi stori cal factors could expl ain the abundance of polyploid clones in arctic areas. During the
las t glaciati on in the late Pleistocene, few zones were free of ice and served as refu ges to
many species in both the anim al and vegetal kin gdoms. Species in those restri cted areas had
to face harsh environme ntal conditi ons that led to the creati on of new geneti c and adapti ve
traits. The development of po lypl oidy mi ght have been fa voured as the ice shee t receded at
the end of the Ice Age and polyploid organi sm s spread to those ne w habitats. Moreover, in th e
Daphnia pulex complex, hi gh gene tic diversity and c lonai di vergence in Arcti c regions has
been ex plained by rec urrent hybridi zation events between different Dap hnia species th at
occ uned in different glac ial refu ges. After the retreat of glac iers, contac t zones between
refugia l races favoured hydridi zation and subsequentl y polyploids formati on (Dufresne and
Hebert 1995 ; Weider and Hobae k 2003).
Asex ual s neither did not have an increased tolerance to environmental ex tremes despite
what has been sugges ted in previ ous studi es (Beaton and Hebert 1988; Parker and Ni kl asson
2000; Schon and Martens 2003; Stenberg, Lundmark e t al. 2003; Kearney 2005). Linkin g
hybridization with parthenogenesis, Kearney (2005 ) sugges ted th at the hi gher heterozygozity
76
of newly formed clones is advantageous in colonizing new habitats and facing new
environmental challenges. Heterozygosity of the clones of this study ran ged from 0.33 to 1
but beared no relationships with tolerance to a range of factors. Nonetheless, some differences
between sexual and asexuals clones of D. pulex have been revealed in our ex periment.
Reproduction mode had a significant effec t on survival of clones at 15001LS. cm' I and pH 5.
However, at pH 5 the pattern is inverted and under the other parameters tested, no signifi cant
difference was observed be tween cyclic and obligate parthenogens. So it is difficult in our
study to attest for an effec t of reproduction mode on the geographic repartition based on
ecologica l tolerance differenti ations. Those results are in concordance with a si mil ar
experimental study of Wei der (1993) that tes ted the effec t of reproduction mode on the
tolerance to therm al and sali nit y stress among Daphnia c lones. He found onl y one signifi cant
di ffere nce effect of breeding mode on salinity tol erance on four repeated experiments, the
asexua\s ex hibitin g a greater to\erance to salinity stress. In hi s three repeated experim ents
with temperature, he also found onl y one significant effect of breedin g mode but then, the
sex uals were more tolerant to thermal stress than the asexual clones. He conc luded that
parthenogens do not have a more broadly genotype tha t could have explained their wi der
geographical di stributi on. Lundmark (2006) also reported !ittle evidence for asexuality as the
main ex planatory factor behind the success of clonai forms of arthropod spec ies with
geographica\ parthenogenesis. Lynch et al. (1989) found a large amount of geneti c variation
in Iife-hi story traits in obligate parthe nogens of D. pulex and argued that assoc iated with the
potential for microevolutionary change within lineages, thi s variation can he lp to explain the
broad geographic di stributi on of the asexual complex. They emph azised that obli gate and
cycl ic parthenogens are not ecologically equivalent and remained that variation in asex uals is
in great part a consequence of the polyphyletic origin of the cl ones as weil as their incompl ete
77
reproducti ve isolati on from their sex ual ances tors as was primaril y hypothesized by Hebert et
al. (1989) and Innes et al. (2000).
T here are two leadin g exploratory models that have been advanced to explai n
geographi cal parthenogenesis. The ' general-purpose' genotype (GPG) refers to the notion that
asex uals may possess a more broadl y adapted genotype . As asex ual genotypes are transmi tted
intac t generati on after generati on, any favourabl e genes compl exes will be selected to face
broad ecologica l demands. On the other side, the Frozen Niche-Vari ati on m ode l (FNV) of
Vrije nh oe k (1 979; 1984) states th at multiple ori gins of asex uals produce an an ay of clones
that capture and freeze genotypic vari ati on of their sex ual ancestors. Then, selection among
clones would favo ur speciali zed genotype havin g minimal niche overl ap with the es tab li shed
clones and th eir sexua l ancestors. Several studies have tested for one or the other mode l on
asex ual organi sms (Weider 1993; Vrijenh oek and Pfeiler 1997 ; VanDoninck, Schoen et al.
2002) . Results do not converge on one model but some lead to the conclusion th at clones are
ge nerali st (GPG) and others th at clones are speciali st (FNV). The main result of thi s study
showed interclonal vari ability in response to acute environme ntal co nditi ons. In ail
treatm ents, survivorship differed among clones. AI so, al! clones showed great vari ation in
their response to the di fferent treatments as demonstrated with the LT
50
va lues. In our results,
no clone seemed to have an equal to lerance to ail treatments. Thi s may agree with the
hypothesis th at cl ones are speciali st, each of them having di fferent ecological demands. That
is in agreement with the study of Weider (1993) mentioned above. Hi s res ults did not support
the noti on that asex ual c lones of D. pulex possess a 'general-pUl-pose' genotypes when
compared with sex ual clones.
78
C lones did not differ in their environm ental tol erance according to their geographi c
origins. Temperatures of the regions covered in thi s study vary greatl y. Mean annual
temperatures are -4.4°C and 12 oC in north ern Québec and in Illinois respectively (data are
from the Gouvernement du Canada website and the NOAA we bsite). One coul d hypothesize
that clones from low latitude regions would have a greater tol erance to hi gh temperature but
no signi ficant effect of latitude (ie, geographic ori gin) was observed am ong clones exposed to
acute temperature. Sorne studi es have demonstrated variation in response to environmental
fac tors am ong c lones that were similar in ploidy level, breeding mode and or geographic
on g1l1.
For
exampl e,
variation
in
response
to
salinity
tolerance
among
obli gate
parthenogenetic clones of D. pulex near Churchill (Manitoba) has been showed. The auth ors
exposed several clones to different salinity conditions and compare their response with the
sa linity they encounter in their ponds. The di stributi on of clones were concordant wi th the
laboratory data suggesting that clones had evolved into ecotypes (Weider and He bert 1987).
Other studi es dem onstrated that local genetic differenti ati on and local adaptation may exp lain
the distribution and the coexistence of clones. LaBerge and Hann (1989) fo und vari ation in
ac ute temperature and oxygen stress among genotypes of D. pulex and could related the
intraspecific differences to the temporal vari ation in environme ntal conditions ex peri enced
directly in the pond they ori gi nated from .
In thi s study, pl oidy level and reproductive mode did not influence clonai survivors hip
un der a wi de ran ge of environmental factors. Hi storical fac tors could however play a
sub stanti al role in the pattern of geographical parthenogenesis. Local adap tation and the
incomplete isolation of reproduction mode are indeed very important in explaining the
ecological differences among clones. Because fre shwater habitats experi enced a wide
fl uctuati on of many environmental conditi ons in time and or in space, clones of Daphnia had
79
to speciali ze in orde r to be able to coloni ze new habitats. The great di versity of clones due to
their polyphyle ti c ori gins has enabled species from the Daphnia pulex compl ex to have a wide
distributi on. Intrac lonal vari ati on in response to ac ute environmental stress have been
observed in thi s study (see standard deviation s in figures l , 2 and 3). It woul d be very
interesting to tes t for the m agnitude of the intraclonal vari ati on in res ponse to di fferent
environmental conditi ons. Phenotypic plasticity may provide m ore ecological flexibility for a
given genotype a nd co uld parti y favo ur the 'general-pupose' genotype hypothesis.
81
CONCLUSION GÉNÉRALE
Le
premIer
objec tif de
cette
étude
était
de
décrire
les
commun autés
zoop lanctoni ques dans la région de Kuujjuarapik et Umiuj aq et de détermi ner l' importance
re lative des process us locaux et régionaux influe nçant la di stributi on des di fférents taxons
identifiés. Les conditi ons environnementales vari aient énormément entre ces étangs. Les
communautés zooplanctoniques des trois types d 'étangs les plus fréquemment rencon trés au
N unav ut (roche ux, thennokarstiques et toundriques) ont é té comparées e t l'infl uence des
fac te urs bi otiques et abiotiques sur ces commun autés a été testée. Se ul es q uatre variabl es
ph ysicoc himiques expliquaient signi ficati vement un quart de la variation dans la compos ition
d 'espèces . La température, la conducti vité, l'altitude et le carbone organi q ue dissous
influençaient la struc ture des commun autés de ces étangs. Il fa ut cependant être prude nt quant
à l' interpréta tion des rés ultats car il s'agit d ' un éc hantill onnage ponc tue l q ui ne représente pas
la structure des commun autés de la saison esti vale de 2007 et l'effet de la température doit
être interpré tée en gardant en tête la grande vari a bilité de ce type de paramètre dans une
même journée et d ' un j our à l'a utre. Enfin, que lques espèces de copépodes et les daphnies du
genre Daphnia domin aient une grande proporti o n des communautés recensées dans la région.
Ces rés ultats concordent avec ceux d 'autres é tudes sur les écosys tèmes aquatiques
subarc tiq ues (W e ider 1987 ; Weider and H ebert 1987; Wil son and Hebert 1992; Rauti o 1998;
Rau ti o 200 1) e t apportent des complé ments d ' inform ati ons sur le ur fonc ti onnement.
Notamment, c'es t à ma conn aissance la première étude de ce type sur les mares
therm okarsti ques malgré l'abondance croissante de ce type d 'étangs dû à la fo nte du
pergéli sol dans ces milie ux. Il s représentent une m anne de nouve ll es ni c hes écologiques pour
différe ntes
espèces
zoopl anctoniques
et
pe uvent
servir
de
véritabl es
laboratoi res
expéri mentaux e n milieu nature l po ur é tudier l'établi ssement des communautés, par exemple.
82
Finalement, cet objec tif a permi s d'établir une base de données sur ces écosys tèmes dans la
région de Kuujjuarapik et d'Umiuj aq. Dans une perspec ti ve de changements climatiques,
cette base de donn ées était nécessaire à un éventuel sui vi des commun autés et des
caractéri stiques physicochimiques et morphométriques de ces trois types d' étangs. É tant
donné le peu d'étude de ce type sur les plans d'eau des hautes latitudes et malgré le ur
abondance et leur sensibilité aux changements climatiques, il es t nécessaire de poursuivre
l'étude du foncti onn ement de ces écosys tèmes et d'échantillonner un plu s grand nombre
d'étangs et une plus vaste zone géographique dans le nord du Québec. De plus, une étude
simil aire mais incl uant un sui vi temporel sur la saison (étangs éc hantill onnés une fois par
semaine durant toute la saison estiva le) contribuerait à préciser nos rés ultats et à agrand ir leur
portée scientifique.
Le deuxième obj ecti f visait à expliquer la di stributi on spati ale et la di versité géné tique
des daphnies du compl exe pulex. Tout d'abord, les connaissances sur la di versité clonale à
l'es t de la baie d' Hudson ont été compl étées . Ensuite, l'évaluati on de l' influence de plusieurs
fac teurs biotiques et abiotiques a été effec tuée dans la région de Kuujjuarapi k et d'Umiujaq
dans le but de comprendre les patrons de di stributi ons de ces génotypes clonaux. Vi ngt-trois
clones ont été recensés et les rés ultats montrent qu ' il s possédaient des préférences
écologiques différentes. La conducti vité, le pH et la profondeur des étangs ex pliquaient une
grande proporti on de la vari atio n dans la di stributi on des clones. Cette varia ti on suggére un e
forte interac tion entre le génotype et l'environnement. Il se peut donc que plus une espèce est
diversifiée génétiquement, plus elle es t tolérante à de grands gradients de facteurs
environnementaux et plus sa di stributi on peut être étendue. Il serait très in téressant d'étudier
la di versité génétique d'autres taxons domin ants de cette région comme certains copépodes et
d'autres cladocères afin de vérifier cette hypothèse. Cette étude apporte également des
83
é léments d'explication sur la distributi o n des daphnies pol yploïdes. L'hypothèse selon
laq ue ll e la polyploïdi e confère une plus grande tolérance aux vari ati ons des conditions
envi ronne mentales n'est pas appuyée par nos résultats. U ne faible conductivité sembl e
favoriser les tripl oïdes qui sont absents des étangs où la conductivité est é levée . Éga lement,
un e plus grande habilité des polyploïdes à compétitionner pourrait ex pliquer le ur dominance
sous de hautes latitudes et la rare coexistence entre les triploïdes et les dipl o·ldes dans ces
étangs. Il reste donc encore bea uco up d' études à effec tuer pour ex plique r la di stributi on
géographique des tripl oïdes . Il fa ut notamment envisager d' autres pi stes de réfl ex ion telles
que l'évitement du prédateur copépode Hesperodiaptomus arcticus qui semble préférer les
eaux à conduc ti vités plus é levées. Éga leme nt, il serait intéressant d 'évaluer la répartition
spatia le des triploïdes de la région de Kuujjuarapik et d'U miuj ak en mes urant la di stance entre
les étangs, notamment, afin de savoir si les triploïdes sont répartis de mani è re sporadiques
dans la région ou s' ils sont concentrés dans des zones spécifiques. L 'éparpillement des
triploïdes comme il sembl e être le cas dans la présente é tude, pourrait évoquer le fait qùe ce
sont des clones qui ne parvi enne nt pas à s'établir et sont excl us compétiti veme nt mais qui
sont réguli èrement renouve lés par l'hybridation de leurs congénères diploïdes du même étang
o u d'étangs voisins. Un sui vi temporel régulier (intra- et intersaisons) des génotypes triploïdes
dans une même région permettrait de vérifier cette hypothèse. Dans le cas de génotypes
sta bles, une meilleure capacité compétitive pounait expliquer le ur présence.
Dans un volet expérimental, deux études sur la ph ysiologie de différents clones de
daphnies ont permis de confirmer les résultats obtenus sur le terrain et d'ouvrir la question de
la di stributi on des clones sur une plus grande éc he ll e spatia le, en utili sant des clones
provenant de la région des grands Lacs (Etats-U ni s et Canada) au Subarctique québécois . Une
première é tude exploratoire a pe rmi s de réfuter l' hypothèse selon laque lle les polyploïdes
84
possède raient une plus grande fle xibilité mé tabolique que les dipl oïdes, ce qui a urait pu
exp liquer le ur grande distribution dans les milieux extrêmes. Une de uxième expérience a testé
la tolérance de plusieurs clones à différents fac te urs ph ysicoc himiques (température, pH et
conductivités) extrêmes. Les rés ultats ont confirmé que les clones de daphnies possédaient
des demandes écologiques différentes mais ni le ni veau de ploïdie, ni le mode de reproducti o n
(sex ué vs asex ué), ni l'ori gine géographique (l atitude) n 'ont e u d ' influence sur la survie des
c lones. E ncore une fois , les polypl oïdes n 'étaient pas plus tolérants aux conditi ons
environnementales adverses que les diploïdes. De la même façon, les asex ués n ' étaient pas
plus tolérants que les sex ués. U ne alternative à l' hypothèse de différences physiologiques
e ntre les sex ués, les asex ués et les ni veaux de ploïdi e pour expliquer les patrons de
distribution géographique des c lones de daphni es reposent sur des facteurs hi storiq ues.
L ' avantage démograp hique de la reproducti on asex uée aurait été favor isé lors de la
colonisation de no uveaux ha bitats lai ssés vacant par le retrait des glaciers lors du pl é istocène.
Une fois les communautés établies, les ancêtres sex ués n 'auraient pas é té capab les de
suppl anter les asex ués. Et finalement, le fait que les asex ués et les po lyploïdes ont des
origines multiples et polyphylétiques chez Daphnia pulex peut expliquer que l' on ne détecte
pas de différences écophysiologiques entre les sex ués et les asex ués, ni entre les ni veaux de
ploïdie. Par contre, les différe nces inter- et intrac lonales confirment que les c lo nes de
dap hnies sont spécia listes et occ upent des niches écologiques différentes que ll e que soit leur
mode de reproducti on et le ur ni veau de ploïdie. Par conséquent, le grand nombre de clo nes, en
d'autres termes la grande di versité génétique des daphnies, pourrait expliquer la di stributi on
cosmopolite de ce compl exe d 'espèces. Il serait pertinent de conduire des études comparabl es
à celle-ci sur la diversité clonale et la répartition des modes de reproduction et des ni vea ux de
ploïdie sur di fférentes espèces ayant des caractéristiques similaires aux daphni es du compl exe
85
pulex afin de valider nos rés ultats ou d'apporter d'a utres pistes de réfl exion sur la
parthénogenèse e t la polypl oïdie géographique.
En conclusion, ce proj et de maîtrise contribue à l' avancement des connaissances sur
les fac teurs responsables de la bi odi versité zoopl anctonique des étangs subarcti ques au ni vea u
inter et intraspécifique.
Non se ulement il
a permi s de décrire
les commun autés
zoopl anctoniques dans la région de Kuujjuarapik et Umiuj aq et de déterminer l'i mpottance
re lati ve des process us locaux et régionaux influençant ces commun autés, mais il apporte de
nouve lles pistes sur la réparti tion des daphnies du co mplexe pulex du subarctique en intégrant
un e approc he descripti ve en milie u nature l e t ex périm entale en laboratoire. De nombreuses
études comme ce proj et seront nécessaires pour palier au manque de conn aissances sur les
écosys tèmes aquatiques subarctiques et arctiques et sur la di stributi on de le ur biodi versité,
que ce soit présentement ou en marge des changements clim atiques. Ces études devraien t
intégrer plusie urs di sciplines compl émentaires comme l'écologie des communautés, la
phys iologie et génétique évoluti ve, comme par exemple dans ce proje t de maîtri se, dans le but
d'élargir la compré hension des patrons de di stributi on des espèces.
86
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