Chytridiomycota Kingdom - Fungi

Chytridiomycota Asexual reproduction by zoospores produced in zoosporangia Zoospores have one posterior flagellum Vegetative thallus variable – range from globose, multinucleate to hyphal forms Growth may be determinate or indeterminate

Zoospores Initially zoospores encyst – withdraw or lose flagellum, rapidly form a cell wall Cyst then germinates to form rhizoids and enlarges

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 anaerobic Some are parasitic on algae, other fungi, aquatic animals, some parasitic on higher plants (crops)

Chytridiomycota Include 5 orders (will discuss 2), distinguished on basis of habitat, zoospore ultrastructure, other characterisitics Chytridiales – mainly aquatic Spizellomycetales – mainly in soils Neocallimasticales – occur in rumen Blastocladiales – mainly aquatic Monoblepharidales – small no of species, all filamentous, unique sexual reproduction

Zoospore ultrastructure

Chytridiales 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

Chytridiales One species is parasitic on amphibians – mentioned in decline of frog populations - Batrachochytrium dendrobatidis

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

Blastocladiales 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)

Coelomomyces life cycle

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

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 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 Zoosporangia

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

Allomyces life cycle

Allomyces Divided into several subgenera based on reductions in the haploid portion of the life cycle, some only reproduce asexually like Blastocladiella

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