Comparative ultrastructural ontogeny of zoosporangia of Zygorhizidium affluens and Z. planktonicum, chytrid parasites of the diatom Asterionella formosa

The fine-structural development of the small, 5–10 m, operculate zoosporangia of the monocentric chytrids Zygorhizidium affluens and Z. planktonicum, growing as epibionts on the planktonic diatom Asterionella formosa, is compared. In both species, following encystment and germination, the germ tube grows over the surface of the diatom frutule and penetrates the host cells by squeezing between the upper and lower girdle lamellae. As the cyst enlarges to form a sporangium, the single lipid globule breaks up and disperses, an operculum differentiates and the nucleus divides. In both species, cleavage of the cytoplasm into zoospore initials is mainly achieved by the infurrowing of the plasmamembrane, although there may be some contribution from internal cleavage vesicles. Flagellar axoneme profiles are not observed in the sporangia until cytoplasmic cleavage is well advanced. Specialized zoospore organelles, such as the rumposome fibrillar vesicles and endoplasmic reticulum-delimited ‘ribosome core’, do not form until the later stages of zoospore differentiation. The taxonomic and functional implications of these observations are discussed.