Zoosporic fungi Kingdom Fungi Zoospores Motile asexual spores

Zoosporic fungi Kingdom - Fungi

Zoospores • Motile asexual spores = zoospores • No cell wall, one flagellum • Flagella – long slender structures extending from cell and surrounded by cell membrane • 9+2 microtubular structure characteristic of eukaryotes

Zoospore • Flagellum anchored in cell with basal body

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

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

Vegetative thalli

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

Zoosporangium formation

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

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 CO 2 concentrations are high • Has been used to examine the biochemistry of differentiation along these two pathways

Blastocladia • Forms polycentric thallus but exhibits determinate growth

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 1 n 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 – 2 n zoospores that encyst and germinate to produce 2 n thallus

Allomyces life cycle • 2 n 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

Allomyces life cycle