D. Rabern Simmons - Chytrid Fungi Online

Thallus Development in the
Chytridiomycota
Rabern Simmons, PEET trainee
University of Maine
Mycological Society of America Annual Meeting
July 29, 2009
Purpose
• Provide a glossary for teachers and students of
the Chytridiomycota, or chytrids
– Slightly different vocabulary than other fungal
groups
– Allow students to accurately explain what they see
– Provide coherent discourse of ideas
To meaningfully construct this
glossary . . .
• Define morphological terms
• Show examples and compare morphologies
• Show evidence that similarity of form does
not necessitate relatedness
– Morphological trends occur in multiple lineages
Chytridiomycota
• True fungi that produce zoospores with single
smooth, posterior flagellum
• Molecular phylogenies hypothesize 2 classes
– Monoblepharidomycetes
• 1 order
– Chytridiomycetes
• 6 described orders
• 3 clades with no ordinal description
Traditional Morphological Taxonomy
• Characters viewed by light microscopy (LM)
– Possession of operculum
• Lid-like portion of zoosporangium
– Amount of thallus devoted to zoosporangium
– Development of zoosporangium from zoospore
cyst
– Number of zoosporangia per thallus
I. Thallus Morphology
Holocarpic
Olpidium sp. (currently incertae sedis)
Eucarpic
Most common chytrid form.
Sparrow, 1960. Aquatic Phycomycetes.
Holocarpic “chytrid”
Zoosporangia
with
zoospore
discharge
tubes
Plant cell wall
Resting
sporangia
of fungus
Olpidium brassicae in root cells (LM). Used with permission of D J S Barr.
Eucarpic chytrids
Rhizoids – Defining feature
Robust and
blunt-ended
Sparse, fine and
tapered
Isodiametric
. . . Just to name a few.
Rhizoids
May have subsporangial swelling, or apophysis
Chytriomyces aureus
on chitin (Hoffman
modulation contrast LM)
JEL72 Unidentified
Cladochytriales on agar
(phase contrast LM)
Unidentified
Spizellomycetales
on agar (LM)
II. Development of the Zoospore Cyst
Endogenous
Exogenous
Zoospore
cyst
JEL369 Clydaea vesicula on agar
(phase contrast LM)
Barr81 Powellomyces hirtus on agar
(Hoffman modulation contrast LM)
Endogenous Growth:
Swelling of zoospore cyst
Zoospore
with
flagellum
Rhizoidal
axis
JEL539 Maunachytrium keaense on agar (phase contrast LM)
Exogenous Growth:
Nuclear migration
Zoospore
cyst (zc)
zc
Zoosporangium
zc
JEL129 Entophlyctis luteolus on agar (phase contrast LM)
Rhizoidal
axis
Exogenous-Endogenous Growth
e.g. Blyttiomyces helicus
Cytoplasm initially exits
zoospore cyst into germ tube
Zoospore cyst
Exogenous
development
Cytoplasm re-enters zoospore cyst
during maturation of zoosporangium
Endogenous
development
Blyttiomyces helicus
Apiculus:
Apex of
initial
zoospore
cyst
On spruce pollen (LM)
III. Number of zoosporangia
Monocentric
Zoosporangium
Polycentric
Papilla
Rhizoids
Rhizomycelium
Rhizoids
JEL 95 Powellomyces sp. on agar
(Hoffman modulation contrast LM)
Zoosporangia
JEL109 Polychytrium aggregatum on agar
(phase contrast LM)
Monocentric Chytrids
•
•
•
•
Holocarpic or Eucarpic
Endogenous or Exogenous
Many Chytridiomycetes
Select Monoblepharidomycetes
– Holdfast – anchor to substrate
– Loss of flagellum
Monocentric Monoblepharidomycete
with holdfast
Harpochytrium hedinii on agar (phase contrast LM)
Emerson & Whisler 1968. Cultural studies on Oedogoniomyces and Harpochytrium,
and a proposal to place them in a new order of aquatic phycomycetes.
Monocentric, Holocarpic
Monoblepharidomycete
Previous
cell wall
Hyaloraphidium curvatum on agar (phase contrast LM)
Polycentric chytrids
• Giving rise to multiple zoosporangia
• Eucarpic and Exogenous
– Rare exceptions
• Several Chytridiomycetes
• Majority of Monoblepharidomycetes
Polycentric Chytrids
Rhizomycelium is coenocytic or rarely septate
JEL109 Polychytrium aggregatum
on agar (phase contrast LM)
JEL610 Cladochytrium sp.
on agar (phase contrast LM)
Pseudosepta in Gonapodya sp.
(Monoblepharidomycetes)
Gonapodya polymorpha on agar (phase contrast LM)
Polycentricity continued
Terminal or intercalary zoosporangia & swellings
JEL109 Polychytrium
aggregatum on agar
(phase contrast LM)
M53 Monoblepharis sp.
On agar (phase contrast LM)
JEL611 Nowakowskiella sp.
on agar (phase contrast LM)
More polycentrics on agar
JEL137 Physocladia obscura
M36 Gonapodya polymorpha
JEL342 Catenomyces persicinus
M6 Gonapodya prolifera
Polycentric Monoblepharidomycete
w/ holdfast & w/o rhizomycelium
Oedogoniomyces spp. on agar (phase contrast LM)
Emerson & Whisler 1968. Cultural studies on Oedogoniomyces and Harpochytrium,
and a proposal to place them in a new order of aquatic phycomycetes.
Problems with morphological taxonomy
1. Morphology in situ not necessarily the same
as pure culture
Lobulomyces poculatus
eucarpic, endogenous, monocentric
“Veil” of
cell wall
Empty zoosporangium
with operculum
on spruce pollen
(phase contrast LM)
“Veil” may be absent
Developing zoosporangium
on agar
(phase contrast LM)
Problems with morphological taxonomy
1. Morphology in situ not necessarily the same
as pure culture
2. Morphology may differ on various nutrient
media
JEL95 Powellomyces sp.
eucarpic, monocentric,
generally exogenous
zc
zc
zc
on agar (Hoffman modulation contrast LM)
zc?
Problems with morphological taxonomy
1. Morphology in situ not necessarily the same
as pure culture
2. Morphology may differ on various nutrient
media
3. Morphology is not indicative of phylogeny
Example 1:
Molecular
clades with
known
monocentric
exogenous
development
Tree adapted from James et al. 2006. A molecular phylogeny of the
flagellated fungi (Chytridiomycota) and description of a new
phylum (Blastocladiomycota).
Example 2:
Molecular
clades with
known
polycentric
growth
Tree adapted from James et al. 2006. A molecular phylogeny of the
flagellated fungi (Chytridiomycota) and description of a new
phylum (Blastocladiomycota).
Conclusions
• Large degree of variation seen in
“simple, basal fungi”
– Makes them interesting, but difficult to explain
• Morphology remains useful for species
identification
– L. poculatus, B. helicus, etc.
• However, morphological similarity is not
necessarily an indication of relatedness
– Must emphasize, or else we are defining but
not educating!!!