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“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,