The Oomycetes are a commercially important class of fungi-like species, because many species are plant parasites that cause extensive damage to crops. There are species adapted to marine or brackish water, but most species are found in terrestrial soil and freshwater habitats (Fig. 1.19). Species grow as parasites on a variety of protist, animal or plant structures, or as saprotrophs on litter. Most species have a heterokont bicili-ated dispersal stage, although some terrestrial species (in the Saprolegniales and Lagenidiales) do not. The anterior cilium has bilateral tubular mastigonemes, but the posterior trailing cilium is smooth. The cilia kinetosome is anchored by a characteristic rootlet. There is an external cell wall of P-glucans (70%) and cellulose (10%), with additional minor components such as mannose. Chitin is not normally found, except in several Leptomitales and Achlya. There are mitochondria with tubular cristae. There is a closed mitosis with an internal mitotic spindle, but centrioles remain outside the nuclear membrane. The growth phase
is diploid, with filamentous branching hyphae into the substrate. Feeding is by secretion of extracellular digestive enzymes in response to substrate, and osmotrophy of dissolved nutrients. Three types of vesicles with glycoproteins are synthesized by the Golgi bodies. One type contains storage material, while the other two are regulated secretory vesicles (Dearnaley and Hardham, 1994). The primary storage material are mycolaminaran and mycolaminaran phosphates (P-glucans). Tip growth requires actin filaments, elongated branched vacuoles, inward Ca2+ fluxes and directional polar transport of cell membrane and wall components. Turgor pressure has been discredited as a mechanism of substrate or host penetration (Money, 1997). The growing hyphae are vulnerable to damage but have a wound response, initiated within seconds. Callose deposition with calcium was demonstrated to form a plug in Saprolegnia ferax (Levina et al., 2000). In response to poor resources or unsuitable habitat, hyphal tips form diploid sporangia that release motile biciliated dispersal cells. Release of the motile cells requires water with low solute concentration and it is inhibited by high external osmotic pressure. The ciliated cells encyst and are stimulated to excyst by nutrients or an appropriate stimulus. The contractile vacuole appears in the ciliated cells and disappears during encystment as the cell dehydrates into a cyst (Mitchell and Hardham, 1999). There are sexual stages with separate male and female gametangia. The haploid female in Achlya secretes the steroid pheromone antheridiol to initiate male antheridia (Carlile, 1996). Haploid nuclei migrate from the male polynucleate antheridia to the female gametes through a fertilization tube. The resulting zygote is a resting cyst which can be dispersed and stimulated to excyst. The role of these species is better known from aquatic habitats, where they are found on decomposing leaves, woody debris, cadavers or other organic matter such as excreta. The order Saprolegniales consists mostly of saprotrophic species with large diameter hyphae (>10 ^m). The morphology of the sporangium and zoospore release are used in determining genera, and the sexual stages are used for species identification. The order Peronosporales also includes terrestrial saprotrophic species, as well as the parasitic Peronosporaceae (downy mildews) and Albuginaceae (white rusts). Other orders are primarily parasitic.
The Hyphochytrea, or hyphochytrids, are a small group with about 25 species, some of which are found in soil (Sparrow, 1973; Karling, 1977).
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