Sporangium differentiation and zoospore fine-structure of the chytrid Rhizophydium planktonicum, a fungal parasite of Asterionella formosa

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