CHARACTERIZATION OF ZOOSPORE AND CYST SURFACE-STRUCTURE IN SAPROPHYTIC AND FISH PATHOGENIC SAPROLEGNIA SPECIES (OOMYCETE FUNGAL PROTISTS)

The importance of the surface structure and chemistry in zoospores and cysts of oomycetes is briefly reviewed and the organelle systems asso ciated with encystment described. The surface structure and chemistry of primary and secondary zoospores and cysts of Saprolegnia diclina (a representative saprophytic species) and S. parasitica (a representati ve salmonid fish pathogen) were explored using the lectins concanavili n A (Con A) and wheat germ agglutinin (WGA) and monoclonal antibodies (MAbs) raised against a mixed zoospore and cyst suspension of S. paras itica. The binding of lectins and antibodies to spores was determined using immunofluorescence microscopy with fluorescein isothiocyanate-la belled probes and with electron microscopy with gold-conjugated probes applied to spore suspensions post-fixation. In bath species Con A, wh ich is specific for glucose and mannose sugars, bound to both the surf ace of primary and secondary zoospores (the surface glycocalyx) and th eir cyst coats and readily induced zoospore encystment. The binding to the cysts appeared to be mainly associated with the matrix material r eleased from the primary and secondary encystment vesicles and which a ppeared to diminish with time. No binding to germ tube walls was obser ved with this lectin. The MAb labelling showed a generally similar bin ding pattern to the primary and secondary cysts to that observed with Con A, although the binding to zoospores was more variable. Primary zo ospores bound the antibodies but secondary zoospores appeared less rea ctive. It is suggested that the MAbs share a common epitope with one o r more of the Con A-binding components. In both species WGA, which is specific for amongst other things the sugar N-acetyl glucosamine, boun d to localised apical patches on the primary zoospores. This lectin al so binds to the ventral groove region of secondary zoospores of S. dic lina, which were induced to encyst by this lectin. In contrast seconda ry zoospores of S. parasitica were not induced to encyst by the additi on of WGA and showed a patchy dorsal binding with this lectin. WGA als o binds to both the inner wall of discharged primary cysts and the you ng germ tube walls of both species. These observations are discussed b oth in relation to other oomycete spores and to their possible functio nal and ecological significance.