SPORANGIUM DIFFERENTIATION AND ZOOSPORE FINE-STRUCTURE OF THE CHYTRIDRHIZOPHYDIUM-PLANKTONICUM, A FUNGAL PARASITE OF ASTERIONELLA-FORMOSA

Sporangium development in the fungal parasite Rhizophydium planktonicu m has been followed by electron microscopy using dual clonal cultures. Following zoospore encystment on Asterionella, a fine germ-hypha is p roduced which penetrates the host via the girdle lamella. The sporangi um develops directly from the cyst and becomes delimited from the germ -hypha by the formation of a septum. The pre-cleavage sporangium devel ops a thickened apical wall (a papilla) which ultimately gives rise to a vesicle which surrounds the emerging zoospores. This pattern of ves icle formation differs significantly from that so far described in oth er chytrid species. The nucleus undergoes mitosis before the sporangiu m cytoplasm becomes cleaved into uninucleate zoospore initials by the formation of an internal furrow system. Specialized zoospore organelle s such as the lipid-associated rumposome and paracrystalline bodies ar e not formed until zoospore initial delimitation is almost completed. The fine-structure of the mature zoospore was used to evaluate the cur rent taxonomic status of this species, particularly in relation to two other chytrid parasites of this host (Zygorhizidium affluens and Z. p lanktonicum). In zoospores of R. planktonicum, the ribosome aggregates typical of this genus are not delimited by endoplasmic reticulum. Zoo spores of this species also contain paracrystalline bodies and dense o cclusions in the region where the flagellum enters the spore body. The se features have previously only been noted in other genera and theref ore it is concluded that this species is probably not closely related to other Rhizophydium spp.