Phylum - Zygomycota, Kingdom Fungi

•      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

•      Two classes:
•      Zygomycetes – a polyphyletic class, the largest (665 spp) and best known class
•      Trichomycetes – smaller (135 spp), less well understood, are commensals on surface and in guts of arthropods

Class - Zygomycetes

•      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
•      Generally divided into 7 orders, we will discuss 3
•      Mucorales – mainly saprotrophs, many to one sporangiospore/sporangium
•      Entomophthorales – mainly parasitic on arthropods, limited mycelium, one sporangiospore/sporangium
•      Glomales – obligate biotrophs, form arbuscuar mycorrhizae

•      Other orders:
•      Kickxellales – produced septate hyphae and modified one spored sporangia
•      Dimargaritales – mycoparasites
•      Zoopagales – parasites of small animals (amoebae, rotifers & nematodes) and fungi including the lethal lollipop,                   Zoophagus)
•      Endogonales – saprotrophs

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
•      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
•      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
•      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
•      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
•      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 (see handout)

•      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
•      Septa form, producing multinucleate gametangia and suspensors
•      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
•      When zygospore germinates, meiosis occurs to start the haploid portion of the life cycle over again
•      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
•      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

Basidiobolus

•      Another fungus that forcibly ejects its sporangiolum
•      Grows on a variety of substrates including frog and beetle dung
•      Sporangiola may be eaten by beetles, which may then be eaten by frog and then grows in dung

Glomales

•      Obligate biotrophs
•      Biotrophic in the roots of higher plants, form arbuscular mycorrhizae
•      Ca 90 spp.
•      Now placed in a separate phylum by some – the Glomeromycota
•      Form tree-like structures = arbuscules in cells of plant – exchange of nutrients between fungus and plant
•      Form large spores in soil – zygospores, azygospores, and chlamydospores depending on species
•      Azygospores
•      Chlamydospores

Phylogenetic relationships (see handout)

Trichomycetes

•      Second class in Zygomycota
•      Small class, contains 3 orders
•      All but a few species are obligate commensals in guts or on exoskeleton of arthropods
•      Found in freshwater (mayfly, stonefly, midges) and terrestrial (millipedes)
•      Thallus is relatively simple – produce a holdfast that anchors them to lining of gut
•      Thallus may be unbranched and aseptate or branched and septate
•      Variety of asexual spores produced (generally 1 spore/sporangium)
•      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, Zygomycota, 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
•      Asexual reproduction – spores produced inside a sporangium – sporangiospores
•      In Chytridiomycota and Oomycota, zoospores produced
•      Transition from multispored sporangia to conidia in both the Oomycota and Zygomycota

Review

•      Kingdom – Protoctista

–  Phylum – Dictyosteliomycota - pseudoplasmodium

–  Phylum – Myxomycota - plasmodium

•      Kingdom – Chromista

–  Phylum – Oomycota – biflagellate zoospores, oospores

•   Order – Saprolegniales

•   Order – Pythiales

•   Order - Peronosporales

•      Kingdom – Fungi

–  Phylum – Chytridiomycota – posterior flagellated zoospore

•   Order – Chytridiales

•   Order – Blastocladiales

–  Phylum – Zygomycota - zygospore

–  Class Zygomycetes

•   Order – Mucorales - saprotrophs

•   Order – Entomophthorales – parasites of arthropods

•   Order – Glomales – arbuscular mycorrhizae

–  Class Trichomycetes – commensals in arthropods

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, Myxomycota