An ultrastructural study of development and reproduction in the nematode parasite Myzocytiopsis vermicola.

An isolate of Myzocytiopsis vermicola, a holocarpic parasite of Rhabditis nematodes, was studied with transmission electron microscopy (TEM) to follow development during infection, asexual and sexual reproduction. Nematodes became infected after attachment of apical cystospore buds to the nematode cuticle. Apical buds were packed with vesicles with dense fibrillar contents, which were absent from the thallus. Some thalli developed into sporangia while others became paired gametangial cells. Zoospore cleavage was often intrasporangial, although during the early stages of an epidemic partially differentiated zoospores usually were released via an exit tube into a fine vesicle. Packets of tripartite tubular hairs (TTH) were not observed in the cytoplasm of either developing or mature sporangia. TEM of sectioned material and whole mounts of zoospores revealed biflagellate zoospores, some without hairs and others with a proximal row of very short hairs on the anterior flagellum. Gametangial contact was via a short, walled fertilization tube and surplus antheridial and oogonial nuclei remained in their respective gametangial cells until disintegration of the periplasm. The mature oospores had a scalloped, electron opaque, epispore wall layer. These observations will be discussed in relation to the likely phylogenetic position of the Myzocytiopsidales within the oomycetes.