Sporangiospore structure and germination in dimargaritales

The structure of dormant and germinated sporangiospores in the mycoparasitic Dimargaritales Dimargaris cristalligena, Dispira cornuta and Tieghemiomyces californicus is described and compared with that found in members of the Kickxellales, Mucorales and Zoopagales. From light and electron microscope observations, the wall of the sporangiospore comprises a thin, electron-dense outer layer which is frequently removed in freeze-fracture preparations and a thicker, fibrillar, electron-lucent layer. The inner surface of the wall bears bacilliform protuberances and the plasmalemma appears correspondingly indented. The tripartite merosporangial wall often persists around the liberated sporangiospore. In D. cristalligena and T. californicus the surface of the sporangiospore bears verrucose ornamentation whereas that of D. cornuta appears smooth. During germination, a new wall layer of variable thickness is deposited between the plasmalemma and the inner surface of the sporangiospore wall. The wall of the germ-tube appears two-layered, the outer layer being contiguous with the inner layer of the sporangiospore wall and the inner layer with the layer synthesized during germination. Cytoplasm of the germinated spore is characterized by a large vacuole containing a voluminous accretion of electron-dense material.