Zoosporic fungi

Kingdom - Fungi

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

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

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

Zoospore ultrastructure

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

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)

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