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Division Chytridiomycota

Asexual Spores, Sexual Spores and Life Cycle

The division is in a state of flux. Recent molecular evidence (James et al, 2006, Nature 433:818-822) indicates the loss of zoospores on more than four occasions, and that zoospores are found on fungi in at least four quite different groups. For convenience, the presence of zoospores will be used as the basis of the division.

The Chytrid fungi are characterised by their asexual state, a zoospore (motile) with a single whiplash flagellum oriented and located posteriorly. The zoospore is formed in a sporangium. Zoospores are released through an opening in the wall, and their release usually indicates the death of the thallus. Note that zoospores with a single tinsel flagellum are found in diverse Protists, though the morphology of some of these microbes is otherwise simialr to some chytrids (eg Hyphochytrium).

In the few species where it has been determined, zoospores of opposite mating type meet, fuse (plasmogamy immediately followed by karyogamy) and go on to form a single resting sporangium. After meiosis, the sporangium germinates to release further zoospores, which may be asexual, in which case, the single zoospore attaches (encysts) to and colonises a substrate. The cyst may be a resting structure or it may grow to form a sporangium. The sporangium may continue growing, it form a resting stage. The thallus may be located on or in the substrate.

In some species, resting sporangia may take the form of a multicelled, thick walled sorus. The sorus is commonly found within parasitised tissue such as a root, where it remains until the surrounding tissue has rotted away. The resting sporangium may arise from haploid or diploid zoospores.

Classes

The Chytridiomycota is the most ancient group of fungi, and is a polyphyletic taxon. Of the fungi, they share most characteristics with the animal kingdom. At present Chytridiomycota consists of a single class, Chytridiomycetes, containing five orders which are separated on ultrastructural and molecular characteristics. The five orders are:

Chytridiales, Monoblepharidales, Neocallimastigales, Rhizophydialies and Spizellomycetales. The differences in morphology of the flagellar roots enable separation of isolates down to the level of genus, and are particularly important for determing the order.

Blastocladiales, formerly part of this Division, remains to be formally placed in its own division. The plant parasite Olpidium also remains to be formally allocated a new division with the Entomophthorales. Rozella, a parasite of the Chytrids, is also awating formal separation from the Chytrids.

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Cultural Conditions

Saprophytic chytrids are commonly trapped from soil or water using pollen, cellulose, chitin, keratin or other complex organic materials. Some can then be cultured on nutrient media containing simple organic energy and protein. Obligate parasites are cultured using their host, inoculation being achieved by washing zoospores onto the host surface. Gut fungi are commonly isolated by dilution, cultures then being maintained in strict anaerobic conditions in a nutrient broth containing cellulose and its breakdown products.

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Ecology

Chytrids require a water film in which zoospores disperse. Thus the fungi can be isolated from habitats that are moist for at least short periods. Chytrids are readily trapped from water and soil, where they may be saprotrophs. Other chytrids may be parasites of plants, animals, algae and other fungi.

Saprotrophic Chytrids appear to use complex carbohydrate as their source of energy. Especially in marine habitats, zoospores are attracted by chemicals released by the host or substrate. Zospores settle on the substrate, and encyst. In some cases, the life cycle may be completed within three days. There appears to be specificity between substrate and fungus, with, for example, Rhizophlyctis rosea and close relations only being trapped on cellulose.

An important group are anaerobic chytrids are found in the gut of herbivores. These fungi are believed to function synergistically in the degradation of carbohydrates in the diet of ruminants. Their attraction to plant materials, capacity to penetrate cell walls and their broad range of degradative enzymes has led to the suggestion that they play an important role in the initial attack of structural carbohydrates. Many gut fungi are obligate anaerobes.

THE REST: The plant parasite Olpidium is unusual among fungi in that the sporangium in the plant cell lacks a host membrane around the fungal protoplast. The fungus also vectors up to 10 viruses, possibly by carrying the virions in fungal cytoplasm. The lack of a host barrier would reduce the capacity of the plant to prevent ingress by the virus. Apart from carrying viruses, few chytrids cause serious plant disease.

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The Thallus

Chytrids lack a true mycelium. Each fungus may be in one of three forms. The simplest is monocentric; a single holocarpic cell. The monocentric cell may be eucarpic, a single cell with or without rhizoids growing into the substrate. The rhizoids lack nuclei. The fungus may form several sporangia on a single rhizomycelium (cells contain nuclei), and the structure is known as polycentric. Each sporangium may release zoospores of different mating types, and release them at different times.

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Further Reading

Alexopoulos, Mims & Blackwell, 1996 Introductory Mycology (4th edition), Ch 4.

Barr DJS 2001 in: The Mycota VII A, Systematics and Evolution , Eds: McLaughlin, McLaughlin and Lemke, Springer, Ch 5.

Powell MJ 1993 Mycologia 85: 1 - 20.

Walker, 1996 Fungi of Australia , Vol 1a, pp 1 - 170.

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