Oomycota and Zygomycota
Phylum: Oomycota
Phylum is similar to Chytridiomycota in
that there are unicellular, short hyphal
to mycelial members and in the latter
mycelium is nonseptate except where
reproductive structures occur.
Differ from Chytridiomycota in that
cell wall component is usually cellulose
and life cycles are always gametic.
Oogamous: Oogonium, with immobile
egg, and sperms in antheridium are
naked nuclei.
Lecture 02
Phylum: Oomycota
Phylum was once thought to represent the
transition from aquatic fungi to terrestrial
fungi, but is no longer thought to be
related to fungi.
In one classification scheme they are
placed in the Kingdom Stramenopila with
Phaeophyta, Chrysophyta, etc. All have
same cell wall composition, zoospore
morphology and food storage material is
mycolaminarin.
mycolaminarin
It is believed Oomycota had lost their
chloroplast (not most parsimonious event).
Order: Saprolegniales
We will go over two orders in this
phylum:
Saprolegniales: Both primary and
secondary zoospores may occur in
the same life cycle. Each oogonium
always produce several eggs.
Peronsporales: Only secondary
zoospore is produced. Each oogonium
always produce one egg.
Zoospores
Chytridiomycota
zoospore
Oomycota primary and
secondary zoospore
Order: Saprolegniales
Life cycle is quite different from
Chytridiomycota. Will use the genus
Saprolegnia as representative of this
order.
Gametic life cycle: Only phylum to
have this type of life cycle.
Sperms are naked, nonmotile
gametes. Only flagellated fungi with
this characteristic.
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Order: Saprolegniales
Summary of
Saprolegnia life
Asexual
Cycle
cycle.
Go cover asexual
cycle first.
Zoosporangia
of Saprolegnia
are not well
differentiated
from hyphae.
Zoosporangia
with primary
zoospores.
zoospores
Zoosporangia delimited
by septum at base and
dense protoplasm.
Order: Saprolegniales
Dense Protoplasm
Order: Saprolegniales
Basal Septum
Terminal Zoosporangium of Saprolegnia
Order: Saprolegniales
Cyst will germinate
to give rise to
secondary zoospore
(laterally
flagellated).
Order: Saprolegniales
Primary zoospores
(posterior flagella)
are released from
zoosporangium.
Encyst
Zoospore swims for a
period of time and
encysts:
encysts rounds up and
cell wall forms around
itself.
Primary
zoospore
Cyst
Order: Saprolegniales
Secondary zoospore
encyst.
Cyst germinates to
give rise to diploid
mycelium.
Secondary
zoospore
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Order: Saprolegniales
Summary of
Saprolegnia life
cycle: Sexual stage.
stage
The sexual stage is
more uniform than
the asexual. Most
species are
homothallic as
shown here.
Sexual
Cycle
Order: Saprolegniales
Formation of
Typically, oogonia
gametangia
and antheridia form
on same mycelium.
Nuclei migrate into
gametangia, followed
by formation of
septa to “trap”
nuclei. Meiosis
occurs at this time. Antheridium
Oogonium
Order: Saprolegniales
Order: Saprolegniales
Fertilization
Tubes
Plasmogamy
Typically, oogonia and antheridia form on
same mycelium.
Order: Saprolegniales
Karyogamy occurs in
oogonium to form
zygotes.
zygotes
The mature zygotes
are referred to as
oospores.
oospores
Oogonia with several eggs is
characteristic of this order. Antheridia
are always difficult to see in live or
prepared material.
Order: Saprolegniales
Oospore germinates
to form the diploid
mycelium.
The mycelium will
give rise to both the
zoosporangia and
gametangia, i.e.,
antheridia and
oogonia.
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Order: Saprolegniales
Variations occur in zoosporangia. Two
common genera:
Saprolegnia: Following release of
zoospores, an internal proliferation
occurs to produce another
zoosporangium.
Order: Saprolegniales
Achyla: Primary zoospores
Encysted
zoospores
are released and encyst,
immediately.
Following release of
zoospores, a new
zoosporangium forms below
the septum and adjacent to
old zoosporangium.
Order: Peronosporales
This order has some of the most well
known pathogens. They include the
genera Pythium and Phytophthora.
Differs from the Saprolegniales in
producing only secondary zoospores in a
zoosporangium that is differentiated
from hyphae, and one egg per oogonium.
Order: Saprolegniales
Proliferation of
zoosporangium
through previous
zoosporangium
Previous
zoosporangium
Order: Saprolegniales
Encysted
zoospores
Achyla empty zoosporangia with encysted
zoospores at tip. Note formation of new,
lateral zoosporangium.
Order: Peronosporales
Zoosporangium of Phytophthora
tropicalis releasing secondary zoospores.
Note that the zoosporangium can readily
be distinguished from hyphae.
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Phylum: Microsporidia
Until the 1990’s this genus was
classified as a very early eukaryote:
Anaerobic, intracellular parasite of
arthropods.
Genus lacks mitochondria.
Was originally assumed to have
evolved before acquisition of
mitochondria.
Based on sequencing of protein tree
of beta-tubulins is a fungus!
Phylum: Microsporidia
Spore germination:
Anchoring Polar tube
disk
ruptured
Pierce
Pierce host
host
cell
cell if
if nearby
nearby
Phylum: Zygomycota
Once thought to be more closely
related to flagellated fungi and
classified in Phycomycetes (literally
algal fungi) with flagellated fungi.
Cell wall material, however, is composed
mostly of chitin. This and other fungal
characters places this taxon with the
fungi, in the strict sense.
Phylum: Microsporidia
Until the 1990’s this genus was
classified as a very early eukaryote:
Placement among other phyla of
fungi uncertain at this point.
Phylum: Zygomycota
This phylum represents the first group
of terrestrial fungi, but still has some
characteristics in common with the
flagellate fungi:
Mycelium is coenocytic.
Although asexual spores are not
motile, they are still produced in a
sporangium.
Above characteristics not found in
other terrestrial fungi
Phylum: Zygomycota
Characteristic that defines phylum is
formation of the zygospore during
sexual reproduction.
Zygospore formation usually results
from the fusion of isogametangia:
isogametangia
gametangia that are morphologically
identical.
The life cycle of Rhizopus stolonifer
will be used as the representative of
this phylum.
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Rhizopus stolonifer Lifecycle
Rhizopus stolonifer Lifecycle
Rhizopus stolonifer life cycle includes
both sexual and asexual reproduction.
Sporangium
development
Stolon
Nuclei migration,
up sporangiophore
Rhizoids
Rhizoids form
with contact
with substrate
Will cover asexual cycle first.
Rhizopus stolonifer Lifecycle
Nuclei migrate
into swollen tip
Developing
Sporangium
Columella forms
trapping nuclei
in future
sporangium
Rhizopus stolonifer Lifecycle
Sporangium
ruptures, releasing
air-borne spores
Germination
Rhizopus stolonifer Lifecycle
Mature
Sporangium
Cell wall forms
around each
nucleus to form
sporangiospores
or just spores
Columella
Rhizopus stolonifer Lifecycle
Asexual reproduction will continue as
long as there is available food.
Sexual reproduction will not take place
unless a different mating strain is
present.
The two mating strains are designated
as “+” (plus) and “-” (minus).
Mycelium
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Rhizopus stolonifer Lifecycle
Two mating strains are allowed to grow
together.
“-” (Minus)
Mycelium
“+” (Plus)
Mycelium
Rhizopus stolonifer Lifecycle
Plus and minus
Progametangia grow
together due to
phermonal response.
Nuclei migrate to
tips of
progametangia.
Suspensors
Gametangia
Sexual reproduction will take place
where mating strains meet in center.
Rhizopus stolonifer Lifecycle
Gametangia fuse,
followed by
plasmogamy and
karyogamy to form
multinucleate
zygote.
zygote
Zygote
Rhizopus stolonifer Lifecycle
Developing
Germination of sporangium
zygospore forms a
sporangium
containing “+” and “-”
sporangiospores.
Germinating
zygospore
Rhizopus stolonifer Lifecycle
Zygote develops a
thick, pitted wall
referred to as the
zygospore.
zygospore
Zygospore has a long
dormancy period
before meiosis and
germination occurs.
Does not disperse!
Zygospore
Zygorhynchus molleri
Sporangiophore
A species that is homothallic and
produces anisogametangia.
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Phylum: Zygomycota
Phylum: Zygomycota
There is a great deal of variation in the
asexual stage of the Zygomycota:
Rhizopus is representative of
simplest and what is believed to be
the earliest sporangia from which
other variations are derived.
Syncephalastrum produces spores in
cylindrical sporangiole.
sporangiole
Phylum: Zygomycota
Cunninghamella produces a one spored
sporangiole and is believed to be the
most recently derived condition.
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