ORIENTATED ZOOSPORE ATTACHMENT AND CYST GERMINATION IN CATENARIA-ANGUILLULAE, A FACULTATIVE ENDOPARASITE OF NEMATODES

Zoospores of the nematode-parasitic Catenaria anguillulae (Chytridiomy cota) were studied by videomicroscopy in sealed films of water on micr oscope slides in the presence or absence of freeze-inactivated nematod es (Panagrellus redivivus). Zoospores swam for more than Ih at a mean velocity of 104 mu m s(-1), interspersed with repeated phases (1-2 min ) of amoeboid crawling on glass or nematode surfaces. They were attrac ted to and encysted near the mouth, excretory pore, and anus of nemato des, or eventually encysted at random on glass and nematode surfaces. The single posterior flagellum was immobile during amoeboid crawling b ut resumed rapid beating when the last pseudopodium was being retracte d. Zoospores encysted by adhesion of the anterior of an amoeboid cell to a surface; then the cell posterior was raised above the anterior so that the flagellum projected perpendicular to the surface, and the fl agellum was retracted by rotation of the cell contents. Cysts germinat ed within 20-60 min by a narrow germ-tube at the site of adhesion. The germ-tube grew a short distance, then formed an intercalary vesicle i nto which the cyst contents emptied by expansion of a cyst vacuole. In several cases the germ-tube penetrated a nematode and formed the vesi cle inside the host. Rhizoids or assimilative hyphae developed from th e vesicle or by growth of the germ-tube tip. An increasing proportion of zoospores that remained motile after 1 h in water films had a globo se body in contrast to the normal elongated form. This seemed to be ca used by damage during repeated transitions between the amoeboid and sw imming phases, because pseudopodia sometimes remained firmly attached to a glass surface. C. anguillulae showed consistent orientation (pola rity) of zoospore encystment and cyst germination. This parallels the behaviour of other zoosporic fungi or fungus-like organisms (Plasmodio phora brassicae, Rozella allomycis, Pythium, Phytophthora and Saproleg nia spp.) suggesting that it is a common feature of zoosporic parasite s. Surface-recognition for encystment by C. anguillulae was mediated b y the zoospore soma, not the flagellum. In addition, we redefine the e arly development of C. anguillulae, including flagellar retraction by rotation of cell contents, non-specific adhesion of zoospores and cyst s to surfaces, and evacuation of cyst contents into a vesicle from whi ch further growth occurs.