The small (about 2 × 3.5 μm) zoospore of the obligately parasitic chytrid Rozella allomycis provides yet another ultrastructural variation on the basic pattern of posteriorly uniflagellate fungal zoospores. This zoospore contains the expected array of organelles: nucleus, mitochondria, "lipid sac" (including lipid globules, a microbody, and a backing membrane), kinetosome, and second centriole. Ribosomes are dispersed throughout the cytoplasm. In the disposition of its microbody Rozella resembles other chytrids, whereas in the occurrence and location of its gamma-like vacuoles it resembles certain blastocladialean fungi. Above all, the zoospore resembles that of Olpidium brassicae. Structural peculiarities of the zoospore include (i) bilateral symmetry; (ii) a helmet-shaped nucleus with extensive membranous projections which surmounts a large, spheroidal mitochondrion; and (iii) a particularly elongate cell shape because of the unusual length (about 1 μm) of the kinetosome and the presence of a 1-μm-long flagellar cavity which surrounds the base of the flagellum. Cytoskeletal elements include cytoplasmic microtubules, microfilaments which mainly reinforce the flagellar cavity, and props which anchor the kinetosome in the plasmalemma at the roof of the flagellar cavity. The role of these elements is discussed. A revision of the description of the olpidiaceous zoospore is suggested.

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Summary. This study examined the behavior of populations of zoospores of the obligately parasitic, endobiotic chytrid Rozella allomycis towards young, vegetative thalli of various saprophytic fungi, in order to identify host-dependent factors which control the development of Rozella. An inverted microscope was employed for continuous observation of parasite-host interaction in petri dishes of broth or agar medium. Two factors appear to control the initial stages of invasion by Rozella of both susceptible and resistant species of the host genus, Allomyces: (i) a soluble exudate which attracts Rozella zoospores, (ii) a receptor on the cell-wall surface which causes Rozella zoospores to adhere, and to encyst and to germinate immediately thereafter. A related, nonsusceptible species, Blastocladiella emersonii, also attracts Rozella zoospores, but supports very limited attachment. Rozella zoospores neither accumulate around, nor adhere to young thalli of non-blastocladialean fungi. This host-specific behavior pattern is compared with that of saprophytic and facultatively parasitic Phycomycetes, whose zoospores show nonspecific chemotactic responses and require no receptor for attachment, encystment and germination.Previous reports on host-parasite relations between the two zoosporic fungi, Allomyces arbuscula Butler (Chytridiomyeetes, Blastoeladiales) and Rozella allomycis Foust (Chytridiomycetes, Chytridiales) concentrated on describing the process by which the individual Rozella zoospore invaded Allomyces. By means of light-and electron microscopy I found that the Rozella zoospore attached to Allomyces, and then it retracted its flagellum, encysted, produced a germtube, and finally injected its protoplast into the host cytoplasm under the pressure of a rapidly expanding vacuole (Held, 1973). I also suggested that penetration into the host cytoplasm depended on localized stimulation of inward growth, or invagination, of the host wall (Held, 1972a). The purpose of the present work was to determine whether host-dependent or host-specific factors governed the initial stage of invasion by Rozdla, and thus delimited the host range, especially whether the zoo-7 Arch. Microbiol., Vol. 95