18 Chytrids

Zoospore
Zoosporic Fungi
Phylum Chytridiomycota
4032604 Lecture 18
Zoospores
Microscopic (2-14 x 2-6 micron),
uninucleate, unicellular, flagellated
spore lacking a cell wall
Formed in a zoosporangium by a process
involving mitosis and cytoplasmic
cleavage
Zoospores do not feed, and rely on
endogenous energy reserves
Flagella (sing. flagellum)
0.25 microns wide, up to 50 microns long
Composed of a 9(2) + 2 arrangement of
microtubules enclosed in a plasma membrane
Attached to a kinetosome
Flagellated centriole; highly conserved structure
composed of 9 triplets of microtubules arranged in
a cartwheel manner
All zoospores have two kinetosomes, but if only
one flagellum is formed, the second kinetosome is
non-functional
Olpidium zoospores/D. J. S. Barr
Kinetosome 9(3) + 0 Structure
Centriole
Nuclear-associated organelle (NAO)
involved in nuclear division—involved
in formation of spindle fibers that
separate chromosomes during division
All eukaryotes with a flagellated stage
in their lifecycle have a pair of centrioles
that replicate during cell division
9 triplets of microtubules
arranged in cartwheel manner
1
9(2) + 2 arrangement of
microtubules in flagellum
triplets
doublets
9 triplets of
microtubules
Section through
kinetosome would
show 9(3) + 0
arrangement
Plasma membrane
Plasma membrane
One doublet of
microtubules in center
9 doublets of
microtubules
9(3) + 0 arrangement of
microtubules in kinetosome
Section through
flagellum would
show 9(2) + 2
arrangement
Flagella
One to many flagella depending on the taxonomic
group
Two types of flagella:
Whiplash
• Smooth, usually directed backwards, propels the zoospore
Tinsel
• With tripartite hairs (mastigonemes); directed forward, pulls
the zoospore
May have only whiplash, whiplash + tinsel, or only
tinsel
Flagellum may be of unequal length (= heterokont)
Zoospore types; from Dick, 2001.
Straminipilous Fungi
Zoospore ultrastructure
Plasma membrane
Lipid globules
microbodies
nucleus
ribosomes
Motile Phase
Requires water
Free-swimming phase is influenced by:
Endogenous energy reserves (lipids)
Environmental conditions
Endoplasmic
reticulum
Microtubules
(cytoskeleton
Kinetosome and nonfunctioning kinetosome
mitochondria
flagellum
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Encystment
Prior to germination, zoospores must:
shed or retract flagella
Form a cell wall
Germination
Direct
Formation of germ tube
Indirect
Formation of another zoospore
Zoosporangium
A typically multinucleate structure that
produces zoospores by a process call
zoosporogenesis
Zoosporogenesis involves mitosis and cleavage of
zoospores from zoosporangium cytoplasm
Zoospores release through one of several
methods:
Breakdown of zoosporangial wall
Opening of cap-like cover called operculum
Discharge papillae plugged with gelatinous
material
Thallus Types
Holocarpic
Conversion of entire thallus into one (monocentric) or more
(polycentric) zoosporangia
Eucarpic
Entire thallus not converted into zoosporangium, and other
structures may be formed:
• Rhizomycelium—hyphal-like structures connecting sporangia,
lack nuclei
• Rhizoids—root-like structures, lack nuclei
• Mycelium
Thallus types relative to
substrate
Endobiotic
Endobiotic
Thallus produced inside host or substrate
Epibiotic
Thallus produced outside host or substrate;
rhizoids anchor thallus to substrate
Epibiotic
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rhizoids
sporangium
Zoosporic fungi
Kingdom Fungi
Phylum Chytridiomycota
Kingdom Straminipila
Phylum Oomycota
Phylum Hyphochytriomycota
Phylum Labyrinthulomycota
Protista
Eucarpic thallus of Spizellomyces
Phylum Plasmodiophoromycota
Phylum Myxomycota
Photo by D. J. S. Barr
Flagellation
Chytridiomycota
1 posterior whiplash flagellum
Hyphochytriomycota
1 anterior tinsel flagellum
Oomycota
2 flagella: 1 whiplash, 1 tinsel
Labyrinthulomycota
2 flagella: 1 whiplash, 1 tinsel
Plasmodiophoromycota
2 whiplash flagella
Phylum Chytridiomycota
True Fungi based on:
Chitinous walls
Flattened mitochondrial cristae
Lysine synthesis by the alpha aminioadipic
acid (AAA) pathway characteristic of all
true Fungi and some protists
• compare to diaminopimelic acid pathway found
in bacteria, plants, and some protists
Myxomycota
2 whiplash flagella
Benny et al. 2001. The Mycota VII Part A. Chap. 6
Classification
123 genera, 900 species in 5 orders:
Chytridiales
Spizellomycetales
Blastocladiales
Monoblepharidales
Neocallimasticales
4
Thallus types
Asexual Reproduction
Chytridiales, Spizellomycetales and
Neocallimasticales
Relatively simple thalli, holocarpic or
eucarpic with rhizoids or rhizomycelium
Blastocladiales
Stalked thalli with rhizoids
Uniflagellate zoospores
One whiplash flagellum inserted in
posterior part of zoospore
Zoospores formed in zoosporangia and
are released through an operculum or
discharge papilla
Monoblepharidales
Filamentous thalli (mycelium)
Zoospore release in Chytrium
Photos by D. J. S. Barr
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Sexual Reproduction
Sexual Reproduction
Plasmogamy can involve one of five
different structures depending on
species:
Plasmogamy and karyogamy results in
formation of resting sporangium
Thickened, often pigmented and/or
ornamented wall
Isogamous planogametes
Anisogamous planogametes
Nonmotile female gamete and motile male
gamete
Gametangial copulation
Somatogamy
zoosporangium
Germination of resting sporangium
occurs after meiosis by cleavage of
cytoplasm into zoospores
Zoospore release
operculum
Order Spizellomycetales
Monocentric thalli
Blunt rhizoid tips (1-2 microns diam)
Inoperculate, multipapillate
zoosporangia
Amoeboid-like zoospores
Mostly soil-inhabiting
rhizoids
germination
ent
encystm
germination
y
is
Plasmogamy by
somatogamy
ios
me
kary
ogam
Young zygote
Resting
sporangium
Order Chytridiales
Zoospore ultrastructure
Monocentric or polycentric thalli
Slender rhizoid tips (< 0.5 micron)
Inoperculate or operculate; if
inoperculate, then single or
multipapillate
Regular-shaped zoospores
Mostly aquatic
Barr, 1990
Spizellomycetales
Chytridiales
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Examples
Potato Black Wart
Synchytrium endobioticum
Black wart disease of potato
Batrachochytrium dendrobatidis
Chytridiomycosis of amphibians
Batrachochytrium dendrobatidis
Responsible for decline of amphibian
populations, particularly frogs, in six
continents (Africa, South, Central and
North America, Europe, Australia and
Oceania)
First reported in 1993
Only member of Chytridiomycota to
parasitize vertebrates
How does it kill amphibians?
Sporangia restricted to keratinized skin of
adult frogs and keratinized mouth parts of
tadpoles
Causes widespread, fatal epidermal infection
only in adults
Epidermal hyperplasia that results may
seriously impair cutaneous respiration and
osmoregulation
Toxin production has not been demonstrated
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Zoosporic Fungi
Phylum Chytridiomycota:
Blastocladiales, Monoblepharidales,
Neocallimasticales
Order Blastocladiales
Zoospores with tightly organized organelles and
characterized by ‘nuclear cap’
Most species are saprotrophs in soil, water, mud,
plant and animal debris; exceptions:
Coelomomyces, is an obligate endoparasite of insects
Catenaria species parasitize small animals
Physoderma species are plant parasites
Separate gametophytic and sporophytic thalli in
several genera, including Coelomomyces, Allomyces and
Blastocladiella
ZOOSPORE ULTRASTRUCTURE
Coelomomyces
Nuclear cap
Alternating sporophytic and
gametophytic stages in mosquito larvae
and copepod (fish lice) hosts,
respectively
Wall-less hyphal bodies (‘hyphagens”)
formed in coelom of host
Blastocladiales
Monoblepharidales
Life Cycle of Coelomomyces
Conjugation of
gametes
Gametophytic
thallus lacking
cell wall forms in
copepod
Motile zygote
encysts, infects
mosquito larvae
Coelomomyces resting spore
Sporothallus
develops in
host, resting
sporangia
formed
Zoospores
(meiospores)
infect copepod
Resting sporangia
Germination of resting spore
Photograph by CC López lastra, JJ García
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Catenaria anguillulae
Catenaria
Polycentric and eucarpic, with catenate
zoosporangia and resting spores that are
connected by rhizomycelium; rhizoids
present
Catenaria anguillulae is a parasite of
nematodes, copepods and other small
animals, but also grows in organic
debris
http://helios.bto.ed.ac.uk/bto/microbes/catenar.htm
Allomyces
Branched thalli with marked bipolarity (basal
rhizoids and apical sporangia)
Sexual reproduction by anisoplanogametes:
Female gametes are hyaline and twice the size of
the orange male gametes
Two different thalli are produced:
Haploid gametophytes
Diploid sporophytes
Order Monoblepharidales
Saprotrophs on substrates (particularly twigs) in
freshwater ponds and streams
Filamentous thallus
Frothy or foamy appearance due to regular arrangement of
vacuoles
Zoosporangia terminal on hyphae
Sexual reproduction oogamous
Nonmotile female gamete (oosphere) formed in oogonium
Motile male gamete (spermatozoid) formed in antheridium
Resting spores with thick, golden, usually ornamented wall
Sporothallus
Top and bottom: gametothallus;
orange male gametangium, larger
hyaline female gametangium
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Female gamete
Male gamete
D. J. S. Barr
a.
b.
c.
Top left: Monoblepharis
thallus with elongate,
terminal zoosporangia and
oogonia
d.
Sexual reproduction in Monoblepharis. a. Non-motile
female gamete and motile male gametes. b. male
gametes swim to female gamete and conjugate (c.).
Resting spore formed after karyogamy (d.) Meiosis
occurs prior to germination of resting spore.
Order Neocallimasticales
Also spelled Neocallimastigales
“Rumen fungi”—first discovered in 1977
Obligately anaerobic chytrids that live in
digestive tract of herbivores (ruminants and
hind-gut fermenters
Some taxa produce polyflagellated zoospores
Zoospores lack mitochondria
Top right: Oogonium and
antheridium
Bottom left: Oospores
http://dogwood.botany.uga.edu/zoosporicfungi/monfoamy.htm
Biology of rumen fungi
Zoospores encyst on plant material in
rumen and intestine
Form thallus with well-developed
rhizoidal system that penetrates plant
material
Passed from mother to offspring,
probably through licking or feces
No known sexual stage
Thallus of rumen fungus
Numbers represent hours after
encystment
Thallus (zoosporangium)
of rumen fungus
Polyflagellated zoospore
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Cow Facts
Each day, the average cow drinks ~ 120 liters
and eats ~ 44 kilos (95 lb) of feed
The rumen has a volume of 100-150 liters
A cow produces 98-190 liters of saliva each
day
A cow produces an average of 25 kilos (55 lbs)
of manure/day
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