Ultrastructure of the Spore Wall in Echinostelium

Abstract

It was recently proposed (Olive, 1970) that the Myxomycetes evolved, via the primitive genus Echinostelium, from the flagellate protostelids. Of the latter, the most likely progenitor was thought to be Cavostelium, which has 1-2-spored fruiting bodies and flagellate cells similar to those of Myxomycetes. Various stages in the life cycle of Cavostelium, especially C. bisporum Olive & Stoianovitch, have been studied under the electron microscope, with considerable attention being given to the ultrastructure of the fruiting bodies and spores (Furtado, Olive, and Jones, 1971). The present study was made to continue the comparison between the two genera. The two species of Echinostelium included in this investigation were E. lunatum Olive & Stoianovitch (1971), the smallest known myxomycete, and the pinkish-spored form of E. minutum de Bary, which occurs commonly on the bark of oak trees in our area. The fruiting bodies of both were treated with the conventional glutaraldehyde-osmium fixation and embedded in Epon-Araldite. Sections were stained with lead citrate and examined with the Hitachi HU-11E electron microscope. The spore wall of both species of Echinostelium was found to be composed of two major layers-an electron-dense outer layer and a nearly electron-transparent inner layer (FIGS. 1, 5-7). In E. minutum (FIG. 1) the inner layer is much wider (250-500 my) than the outer (ca. 100 m,u), but in E. lunatum (FIGS. 6, 7) the two layers tend to have more nearly the same width. Under higher magnifications the outer layer in both species reveals a conspicuous fibrous network (FIGS. 2-4, 7). In earlier stages of development it appears as a homogeneous electron-dense component, but in E. lunaturn, as the outer layer matures,