The fine structure of the zoospore of Sorochytrium milnesiophthora

Abstract

As a part of an investigation of Sorochytrium milnesiophthora Dewel, a parasite of the tardigrade Milnesium tardigradum, the ultrastructure of the zoospore was examined. In overall organization, the zoospore is similar to the spores of the Blastocladiales. The nucleus is conical and located in the posterior third of the cell. A large nuclear cap is anterior to the nucleus. In an arrangement unique for members of the Blastocladiales, the nuclear cap contains, in addition to aggregated ribosomes, a single complex mitochondrion, lipid globules, and microbodies. Portions of the nuclear cap extend posteriorly and partially surround the kinetosome. The kinetosome has nine anterior rootletlike projections that link it to the mitochondrion and nine posterior kinetosomal props that attach it to the plasmalemma. It lies at the base of the nucleus as an anchor for a single, posterior whiplash flagellum. A nonkinetosomal centriole is positioned at an approximate 45° angle to the kinetosome. In the peripheral cytoplasm, there are three structurally distinct types of inclusions hypothesized to be adhesion vesicles, phosphate storage bodies, and gamma-like particles. Therefore, the ultrastructure of the zoospore supports the inclusion of this fungus in the Blastocladiales. Key words: Blastocladiales, zoospore, ultrastructure.