Electron microscopic study of oospore maturation and germination in an emasculate isolate of Saprolegnia ferax. 2. Wall differentiation

The fine structure of oogonium and oospore walls of Saprolegnia ferax and the changes associated with germination are described. Mature oogonium walls have well defined primary and secondary layers with pits arising as the result of the localized absence of secondary wall deposition. The mature oospore wall has a 0.1 μm thick outer wall separated from a 2.0 μm thick inner wall by a thin (40 nm) electron-dense, multilaminate layer. Within the inner wall is an optically dense (0.5 μm) inner zone composed of particulate and membranous material. At the onset of germination most of the microfibrillar inner wall is digested and reabsorbed and a new germination wall is secreted around the protoplast. As the spores swell the vestigial oospore remnants either rupture or disintegrate and germ tubes develop by the outgrowth of the germination wall. Elaborate plasmalemmasome complexes appear during the period of spore swelling and the mature germling walls become thickened and contain abundant lomasomes and peglike ingrowths. Oospore wall morphogenesis in Saprolegnia is compared with that in other oomycetes and discussed in relation to the possible mechanisms of dormancy.