zoosporic fungi kingdom - fungi. zoospores motile asexual spores = zoospores no cell wall, one or...
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Zoospores• Motile asexual spores =
zoospores• No cell wall, one or two
flagella• Flagella – long slender
structures extending from cell and surrounded by cell membrane
• 9+2 microtubular structure characteristic of eukaryotes
Zoosporic fungi• Asexual reproduction by
zoospores produced in zoosporangia
• Fungal zoospores have one posterior flagellum
• Vegetative thallus variable – range from globose, multinucleate to hyphal forms
• Growth may be determinate or indeterminate
Zoospores• Produced in
zoosporangium• Swim away• Zoospores encyst –
withdraw or lose flagellum, rapidly form a cell wall
• Cyst then germinates to form rhizoids and enlarges
Zoosporic fungi
• Divided into 3 phyla
• Chytridiomycota (706 spp)
• Neocallimastigomycota (20 spp)
• Blastocladiomycota (179 spp)
Chytridiomycota
• Habitats – zoospores require free water in which to swim – many occur in aquatic habitats, also found in soil water
• Many species are saprotrophic – grow on a variety of substrates, most are aerobic,
• Some are parasitic on algae, other fungi, aquatic animals, some parasitic on higher plants (crops), one is parasitic on frogs
Chytridiomycota• Includes 4 orders, distinguished on basis
of habitat, zoospore ultrastructure, other characterisitics
Chytridiales – mainly aquatic• Spizellomycetales – mainly in soils• Rhizophydiales -• Monoblepharidales – small no of species,
all filamentous, unique sexual reproduction
Chytridiales – “chytrids”• Primarily aquatic• Saprotrophs grow on variety of substrates
– “baiting”• Parasites of algae, fungi, animals, higher
plants –• “black wart of potato” caused by Synchytrium
endobioticum
• Olpidium brassicae is a cabbage parasite that is a vector for a plant virus
Sexual reproduction
• Great deal of variation, but nuclear events, e.g. meiosis, not clearly determined
• Fusions have been seen between zoospores, gametangia, rhizoids
Vegetative thallus
• Single multinucleate thallus with no appendages
– If grows within host cell it is endobiotic
– If entire thallus is converted to zoosporangium, it is holocarpic
Vegetative thallus• Many species form rhizoids –
tapering structures that anchor thallus and increase surface area for absorption of nutrients
• During differentiation, the entire thallus is not converted into a zoosporangium – eucarpic
• May be within host cell – endobiotic or outside - epibiotic
Vegetative thallus• Some chytrids
produce only one zoosporangia per thallus – monocentric
• Others produce multiple zoosporangia – polycentric
• Produce rhizomycelium
Zoosporangia• Thallus (or part) differentiates into
zoosporangium• Triggered by environmental conditions,
thallus size, nutrient concentration• Multinucleate cytoplasm is cleaved into a
number of zoospores• Golgi produce vesicles that are deposited
around nuclei – form plasma membrane, flagella
• Once formed zoospores escape sporangium
Zoosporangium
Zoospores are released from zoosporangia by
• Breakdown of sporangium wall
• Forming 1 or more discharge papillae
Opening in papilla may be • A lid = operculum• By becoming thin and
dissolving - inoperculate
Resting spores
• Chytrids may form resting spores – thick cell wall, may be ornamented with spines, knobs or may be smooth
• Typically undergo a period of dormancy
Batrachochytrium dendrobatidis• Parasitic on amphibians – colonizes the
epithelium of adult frogs –causes a fatal inflammatory disease
• responsible for decline of frog populations -
B. dendrobatidis
• The disease only discovered in 1998
• Very low specificity for frog species
• The chytrid also infects tadpoles – in mouthparts but does not kill them
Monoblepharidales
• Small order of filamentous eucarpic thalli that produce zoosporangia on ends of hyphae
• Produce small motile male gametes and large non motile female gametes
• Thought that life cycle is haploid, i.e. germination of zygote includes meiosis
Neocallimasticomycota• Occur in rumen and hindgut of mammalian
herbivores, also in anaerobic aquatic environments
• Morphologically similar to chytrids
• Degrade lignocellulose, ferment glucose to acetate, lactate, ethanol & hydrogen
• Are obligately anaerobic – no mitochondria, have hydrogenosomes
• Monocentric or polycentric, zoospores uniflagellate or multiflagellate
Blastocladiomycota• Relatively small order – mainly
saprotrophs, great variation in vegetative thallus
• Characteristics– Produce brown, thick-walled pitted resting
sporangia– Characteristic zoospore (nuclear cap
containing cellular ribosomes)
• Representative genera
Coelomomyces
• Obligate parasite of aquatic animals – diploid phase on mosquito and midge larvae, haploid phase on copepods
• Forms a holocarpic, endobiotic thallus • Forms isogametes that are motile for
sexual reproduction• Possible biological control agents for
mosquitoes (importance in understanding life cycles)
Blastocladiella• Monocentric thallus, eucarpic• Asexual life cycle – can form two
types of sporangia depending on environment– Thin walled zoosporangia– Thick walled resting sporangia when
CO2 concentrations are high
• Has been used to examine the biochemistry of differentiation along these two pathways
Allomyces• Great deal of research on development
and genetics
• Some species reproduce both sexually and asexually, in others only asexual reproduction
• Some species exhibit a haploid – diploid life cycle– Haploid vegetative mycelium– Diploid vegetative mycelium
Allomyces
• Haploid and diploid mycelia are identical except for the reproductive structures they produce
– Haploid mycelium produces gametangia
– Diploid mycelium produces zoosporangia and resistant sporangia
• Hyphae branch dichotomously, produce septa with many perforations
Allomyces life cycle• Haploid zoospore
germinates to form 1n thallus
• Tips of hyphae produce male and female gametangia
• Male gametangia orange
• Female gametangia colorless
Allomyces life cycle
• Cytoplasm in gametangia cleaves to produce gametes
• Both gametes are motile, leave gametangia through discharge pores in papillae
Allomyces life cycle• Gametes swim
– Male gametes smaller, orange
– Female gametes larger, colorless
• Female gametes produce substance, sirenin that attracts male gametes chemotactically
• Male and female gametes fuse (plasmogamy and karyogamy) to form zygote
Allomyces life cycle
• Zygote swims and encysts
• Germinates to produce diploid mycelium
• Produces zoosporangia – 2n zoospores that encyst and germinate to produce 2n thallus
Allomyces life cycle
• 2n mycelium also produces resistant sporangia – thick walled, pitted, brown structures that can remain dormant
• When resistant sporangia germinate, they undergo meiosis to form haploid zoospores that start the cycle over