“Zygomycota”

Kingdom Fungi

“Zygomycota”

• Vegetative stage – well developed aseptate hyphae

• Asexual reproduction by nonmotile sporangiospores

• Sexual reproduction – Zygospore produced in a zygosporangium from fusion of two similar gametangia

• Ca. 800 spp.

“Zygomycota”• Is polyphyletic has been divided into four

subphyla

• Will discuss major orders– Mucorales – saprobic (420 spp)– Entomophthorales – parasites of arthropods

(240 spp)– Harpellales & Asellariales (Trichomycetes) –

smaller (135 spp), less well understood, are commensals on surface and in guts of arthropods

Mucorales

• Zygospore production is generally similar among species, therefore classification is based on characteristics of asexual reproductive structures

• Asexual reproduction is typically by production of sporangiospores, but we will see lines of evolution in which the number of spores/sporangium is reduced until there is only 1 spore/sporangium - conidium

Mucorales• Grow saprotrophically on decaying

plant and animal remains in soils, dung, etc.

• Produce large numbers of asexual spores that are dispersed in the air

• Common contaminants in laboratory• Some are important in spoiling food

– common bread mold, storage diseases of fruits and vegetables

• Some infect humans and animals – opportunistic pathogens

Mucorales

• Typically form aseptate hyphae, septa formed to delimit reproductive structures

• Some species form rhizoids

Multispored sporangium

• Morphology of sporangia varies, basis for classification

• Typical multispored sporangium contains

• Sporangium wall• Columella• sporangiophore

Development of sporangium

• Tip of sporangiophore swells

• Swelling increases, contains multinucleate cytoplasm

• Cytoplasm is cleaved to form all spores at one time – cell membrane and cell walls laid down around nuclei

Development of sporangia

• After formation, sporangial wall may break and release spores into the air or

• A drop of fluid may envelop the sporangium with spores being dispersed by small animals that touch the sporangium

Reduction of sporangium

• Several lines of evolution in the reduction of number of spores/sporangium

• Examples of modifications of sporangia

• Thamnidium – both multispored sporangium and smaller sporangia = sporangiola

Reduction of sporangia

• Blakeslea – sporangiola with 3 spores/sporangiolum

• Cunninghamella – one spore/sporangiolum, spore wall and sporangiolum wall indisdistinguishable except with EM

Reduction of sporangia

• One group in the Mucorales produce cylindrical sporangia = merosporangia

• Also see a reduction in the number of spores/sporangium in this line

• Syncephalastrum

Other sporangial modifications• Pilobolus – fungal

shotgun• Grows on herbivore

dung• Sporangiophore

contains carotenoids, acts as a lens to focus light and direct the growth of the sporangiophore to point the sporangium at light source

Pilobolus• As the sporangium and sporangiophore

mature, the sporangiophore builds up a very high turgor pressure

• Sporangium as a whole is shot off suddenly, directly at the light – up to 3 m

• A drop of fluid on the sporangium causes it to stick to whatever it hits

• Phototrophic ability allows sporangium to be shot out of a pile of dung, land on grass, be eaten by herbivores

Pilobolus• Passage through gut of herbivore activates spore germination

• Sporangium is dark in color to prevent damage from UV when attached to grass

Sexual reproduction

• Involves fusion of two multinucleate gametangia that are similar in structure, may differ in size

• Gametangia are produced as terminal swellings of hyphal branches

• After plasmogamy – a thick walled zygospore is produced with a zygosporangium

• Both homothallic and heterothallic species

Life cycle

• Vegetative mycelium is haploid, reproduces asexually by producing sporangiospores in sporangia

• In a heterothallic species, when two compatible strains come together, hyphal branches form, enlarge to form progametangia

Life cycle

• Septa form, producing multinucleate gametangia and suspensors

Life cycle

• Plasmogamy occurs, end walls of gametangia dissolve and cytoplasm of gametangia mixes

Formation of zygospore

Life cycle

• Zygospore forms inside zygosporangium

• Zygospore develops thick wall, warty appearance, dark in color

• Karyogamy occurs at different times in zygospore formation in different species, so zygospore is diploid at some point

Life cycle

• When zygospore germinates, meiosis occurs to start the haploid portion of the life cycle over again

Life cycle

Variation in zygospores

• Some variation is seen in zygospore formation

Entomophthorales

• Arthropod parasites, 240 spp.• Asexual reproduction by one

spored sporangiola (conidia)• Mycelium exhibits limited growth

in the body of the host, forms septa and fragments

• Entomophthora is a parasite of flies – seen when fly is stuck to window, white halo around it

Entomophthora

• Mycelium fills up body of fly, forms sporangiophores that extend out of segments of abdomen

Entomophthora

• Sporangiophore builds up pressure, shoots off sporangiolum when there is air movement (another fly)

• If misses, can form another structure that shoots it off, up to 3 times

• Sexual reproduction not well understood, form resting spores that are similar to zygospores

Entomophthorales

Glomeromycota

• Obligate biotrophs

• Biotrophic in the roots of higher plants, form arbuscular mycorrhizae

• Ca. 169 spp.

Glomerales

• Form tree-like structures = arbuscules in cells of plant – exchange of nutrients between fungus and plant

• Form large spores in soil –azygospores, and chlamydospores depending on species

Glomales

• Azygospores• Chlamydospores

Phylogenetic relationships

Trichomycetes

• All but a few species are obligate commensals in guts or on exoskeleton of arthropods

• Found in freshwater (mayfly, stonefly, midges) and terrestrial (millipedes)

Trichomycetes

• Thallus is relatively simple – produce a holdfast that anchors them to lining of gut

Trichomycetes

• Thallus may be unbranched and aseptate or branched and septate

• Variety of asexual spores produced (generally 1 spore/sporangium)

Trichomycetes

• Sexual reproduction involves formation of zygospores (in one order)

• Fusion of cells in thallus

• Formation of zygosporophore and then zygospore

“Lower fungi”• Major groups of lower fungi –

Chytridiomycota, Blastocladiomycota, Neocallimastigomycota, “Zygomycota”, Glomeromycota (Oomycota)

• Exhibit diversity in vegetative thallus, asexual and sexual reproduction

• Mycelium when present is typically aseptate

• Used to be placed in one class = “Phycomycetes” – now clear that they do not share a common phylogeny

“Lower fungi”

• Asexual reproduction – spores produced inside a sporangium – sporangiospores

• In Chytridiomycota, Blastocladiomycota, Neocallimastigomycota, and Oomycota, zoospores produced

• Transition from multispored sporangia to conidia in both the Oomycota and “Zygomycota”

Reproduction

• Asexual - Sporangia – produce sporangiospores, in some zoospores

• Sexual Life cycles• Haploid life cycles – Zygomycota produce

zygospore, isogametangia• Diploid life cycles – Oomycota produce

oospore, oogonia and antheridia• Haploid – Diploid life cycles – Allomyces,