L. 6-G. Biology Mycology D. Ebtihal Muiz Phylum Zygomycota, Kingdom Fungi




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L.6-G.Biology Mycology D.Ebtihal Muiz

Phylum - Zygomycota, Kingdom Fungi

Characteristics

Zygomycota, like all true fungi, produce cell walls containing chitin. They grow primarily as mycelia, or filaments of long cells called hyphae. Unlike the so-called 'higher fungi' comprising the Ascomycota and Basidiomycota which produce regularly septate mycelia, most Zygomycota form hyphae which are generally coenocytic because they lack cross walls or septa. There are, however, several exceptions and septa may form at irregular intervals throughout the older parts of the mycelium or are regularly spaced in two sister orders of Zygomycota, the Kickxellales and Harpellales.

The unique character (synapomorphy) of the Zygomycota is the zygospore. Zygospores are formed within a zygosporangium after the fusion of specialized hyphae called gametangia during the sexual cycle . single zygospore is formed per zygosporangium. Because of this one-to-one relationship, the terms are often used interchangably. The mature zygospore is often thick-walled , and undergoes an obligatory dormant period before germination. Most Zygomycota are thought to have a zygotic or haplontic life cycle (Figure 1). Thus, the only diploid phase takes place within the zygospore. Nuclei within the zygospore are believed to undergo meiosis during germination, but this has only been demonstrated genetically within the model eukaryote Phycomyces blakesleeanus (Eslava et al. 1975)
 

Figure 1. Generalized life cycle of Zygomycota. Asexual reproduction occurs primarily by sporangiospores produced by mitosis and cell division. The only diploid (2N) phase in the life cycle is the zygospore, produced through the conjugation of compatible gametangia during the sexual cycle

 

Zygomycota typically undergo prolific asexual reproduction through the formation of sporangia and sporangiospores. Sporangiospores are distinguished from other types of asexual spores, such as conidia of the Ascomycota and Basidiomycota, by their development. Walled sporangiospores are formed by the internal cleavage of the sporangial cytoplasm. At maturity, the sporangial wall typically disintegrates or dehisces thereby freeing the spores that are usually dispersed by wind or water.



Sporangia are formed at the ends of specialized hyphae called sporangiophores. In the model organism, Phycomyces blakesleeanus, sporangial development has been studied extensively to understand the genetic basis for various trophisms, including the strong phototrophic responses to blue light. A unique spore dispersal strategy for the Mucorales is exhibited by the dung fungus Pilobolus, whose name literally means 'the hat thrower'. The entire black sporangium is explosively shot off of the top of the sporangiophore up to distances of several meters. Phototrophic growth of the sporangiophore facilitates dispersal away from the dung onto a fresh blade of grass where it may be consumed by an herbivore, thereby completing the asexual cycle after the spores pass through the digestive system. Some members of the Entomophthorales (e.g., Basidiobolus, Conidiobolus) also reproduce via forcibly discharged asexual spores. Interestingly, species of Basidiobolus, Conidiobolus and several other genera produce a second kind of spore on a long stalk that appears to have certain morphological adaptations for efficient insect dispersal.

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
•      Septa form, producing multinucleate gametangia and suspensors
•      Plasmogamy occurs, end walls of gametangia dissolve and cytoplasm of gametangia mixes




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


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


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


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








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