Fig. 1.22. Typical morphological stages of a chytrid. (A) Cyst and monociliated zoospore dispersal phase. (B) Extending branched thallus from a cell growing on a substrate. (C) Enlarged chytrid with numerous spores, each holding a zoospore, about to discharge. Scale bar 25 ^m.
darting cells which change direction quickly. The monociliated cell can be partially amoeboid when on a surface or thin water film. Chytrids are ubiquitous as a group, found from the arctic to tropical climates. They have been most studied in aquatic habitats (Sparrow, 1960; Powell, 1993). They are very common in soil samples but are usually overlooked. Soil species are also found active in bogs, riparian and stream habitats. Many species occur on microinvertebrates and on aerial parts of plants. Some species are parasitic on a variety of animals, fungi and plants, including commercially important crops and domestic animals. Many soil species are predatory on microinvertebrates (nematodes, rotifers, tardigrades and eggs), on larger invertebrates and on protists, including fungal spores and other chytrids. Most soil species are sapro-trophs on pollen, chitin, keratin, cellulose and substrates that are difficult to digest. Their role in facilitating the digestion of these compounds is largely ignored. It is believed that they facilitate the entry of other organisms, including bacteria and fungal hyphae, into the litter enclosed by these protective polymers by loosening and softening the protective walls. For a general introduction to the literature, students are referred to Fuller and Jaworski (1987), Powell (1993), www.botany.uga.edu/ zoosporicfungi/ and www.botany.uga.edu/chytrid/
Structurally, the cell wall of chytrids consists of chitin and P-glucan polymers, similar to other fungi. The mitochondria have flattened cristae, and the storage products are glycogen and lipid vesicles. Several organelles are particular to this group. There is a nuclear cap of dense ribosomes with endoplasmic reticulum closely associated. There is a microbody-lipid globule complex at the cell posterior, which could be involved in lipid metabolism. Adjacent to this microbody, the rumpo-some consists of flattened membrane vesicles which have been implicated in sensory orientation and taxis in certain species. The rumposome often has bridge connections to the cell membrane and to the kinetosome microtubular rootlet. It stores Ca2+ ions which may mediate the direction of taxis. A cytoplasmic centriole exists in addition to the basal body of the kinetosome. Sex probably occurs in most species and involves male and female gametes produced in separate antheridia and oogonia on terminal hyphae. Motile gametes are released and their conjugation involves pheromones.
Chytrids find their prey or substrate by chemotaxis towards specific molecules (see list in Sparrow, 1960). The initial detection is from leakage of soluble nutrients from dead cysts, spores, litter cells and decomposing tissues. Once detected, changes in the frequency of turning and distance of forward movement (kinesis) direct the cell towards the source. The activity of motile cells varies with moisture, temperature, soil solution composition and amount of stored food reserves. Upon making contact with substrate, its recognition and attachment involves surface molecules and strong binding to prevent accidental detachment. The motile cell (zoospore) encysts and loses the cilium, and grows cytoplasmic extensions into the substrate or prey. The cytoplasmic extensions (thallus) resemble hyphae and can be extensively branched. The growing thallus is not the result of cell divisions, does not have a cell wall as fungal hyphae would, and is capable of phagocytosis at least in some species (Powell, 1984), as well as extensive secretion of extracellular digestive enzymes and osmotrophy. When the food resources are exhausted, dispersal spores (or resistant cysts) are formed by repeated mitotic divisions. Release of the spores occurs at an opening away from the substrate and thallus. It may require rehydration after desiccation and sometimes partial decomposition of the chitinous wall by bacteria. The emerging motile cells (zoospores) can encyst several times until a suitable substrate for attachment and thallus growth is found. The cysts of chytrids are very resistant to long periods of desiccation, chemical attack and anaer-obiosis. This is particularly bothersome with parasitic species which can reappear after decades, from infected soil.
Many of the taxonomic characteristics for preliminary investigation are obtained by light microscopy with Nomarski optics. Based on the ultrastructure of the motile cells (zoospore), morphology, development and ecology, the four orders are separated into two lineages (Barr, 1983) (Table 1.4). The Chytridiales and Monoblepharidales contain mostly (but not exclusively) aquatic genera, whereas the Spizellomycetales and Blastocladiales are primarily soil species. For example, Chytriomyces (Chytridiales) can be isolated in soil and freshwater samples where they digest chitin. The Monopblepharidales, although aquatic, are found in decomposing insect parts, plant litter and woody debris, similar to the habitat of their soil counterparts. The Spizellomycetales are particularly adapted to soil (e.g. Entophlyctis, Karlingia, Rhizophlictis, Rozella and Spizellomyces). The motile stage lacks the ribosome cluster, and the rumposome is absent, with microtubules from the kinetosome oriented more randomly and associated with the nucleus.
Table 1.4. The organization of the Archemycota phylum in the fungi. Phylum Class Order
Archemycota Chytridiomycetes Chytridiales, Monoblepharidales, Spizellomycetales,
Neocallimastigales Enteromycetes Eccrinales, Amoebidiales Allomycetes Blastocladiales, Coelomomycetales Bolomycetes Basidiobolales Glomomycetes Glomales, Endogonales
Zygomycetes Mucorales, Mortierellales, Dimargaritales, Kickxellales,
Piptocephalacea, Cuninghamellales Zoomycetes Entomophthorales, Zoopagales, Harpellales, Asellariales, Laboulbeniales, Pyxidiophorales
The Blastocladiales (Allomycetes) are active in fresh water and moist soils (e.g. Blastocladia and Blastocladiella). The metabolism of many sapro-trophic Blastocladiales species is fermentative and carboxyphilic, producing lactic acid as an end-product. Characteristically, motile cells aggregate the organelles in a complex in one part of the cytoplasm. Germination from the inactive spores is bipolar, with each emerging hypha repeatedly branching into finer hyphae to produce the mycelial thallus.
Although the literature refers to infections and parasitism when discussing chytrids, in many cases they are predacious and osmotrophic. The distinction between parasites and predatory species can be made by looking at the invading thallus. In cases of parasitism, the chytrid and host membranes remain intact and separate the two cytoplasms. In all other cases, the invading chytrid aims to destroy and digest the prey. Once a host cell is penetrated, lysis occurs within seconds. For example, Sorochytrium milnesiophthora is saprotrophic on dead rotifers, nematodes and tardigrades, but it is also predatory and capable of penetrating the cuticle of living individuals. The prey eventually dies and is digested, until release of motile zoospores propagate the individual to other prey or substrate in the microhabitat. Saprotrophic species are attracted to adequate substrates in the litter. The thallus grows into micro- and macrodetritus. The Chytridiomycetes provide good examples of a taxon with intermediate species between free-living species, opportunistic and obligate parasites.
The Enteromycetes are endosymbiotic chytrids in the intestinal tract of animals, found in ruminants. They are anaerobic species with a broad spectrum of digestive enzymes to digest plant cell walls and lignocellu-losic debris. The chytrid thallus grows into the chewed macrodetritus from ingested plant debris.
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