Ultrastructural Studies of Protostelids: The Fruiting Stage of Cavostelium Bisporum

SUMMARY

The morphology of the sporocarp of the protostelid Cavostelium bisporum was studied by both scanning and transmission electron microscopy. Sporocarps for sectioning were fixed with sequential glutaralde-hyde-OsO₄.

The stalk of the sporocarp is located on a supporting basal disc at the point of convergence of numerous fine wrinkles, and terminates in a distinct apophysis to which a spore pair is attached. The spore wall is ornamented with blunt projections. In sectioned material the wall appears more or less uniformly thickened and traversed by numerous tubular cylindric thickenings that extend into fingertip-like projections observable on the spore surface. Members of a spore pair are easily separated and each has a circular scar consisting of a peripheral ring with small projections at its inner edge and an inner area of various sized patches of projections. In section, the adjacent spore walls are seen to be connected by delicate structures equivalent to the projections observed on the scar.

Each protoplast of a spore pair encysts and forms additional wall material adjacent to the plasmalemma. The cytoplasmic organelles and nuclear structure are essentially similar to those of the amoebo-flagellate and cyst stages. There is a reduction in membrane structures. The pair of centrioles in each protoplast persists in either an end-to-end or V-shape arrangement.

The sporocarp is surrounded by an expandable evanescent sheath, which is more clearly observed around the stalk of Protostelium mycophaga var. crassipes.