Electron microscopic study of oospore maturation and germination in an emasculate isolate of Saprolegnia ferax. 1. Gross changes

Abstract

The main morphological and cytological changes which accompany oospore maturation and germination in an emasculate isolate of Saprolegnia ferax have been followed by light and transmission electron microscopy. Oospore development proved similar to that described in antheridiate species except for the absence of motile granules within the central ooplast vacuole. Germination followed within a few days of maturation although it did not occur synchronously within a single oogonium. The ending of dormancy is indicated by a thinning of the oospore wall and a decrease in cytoplasmic refractivity. A new germination wall is secreted around the protoplast and the contents of the central ooplast break down and are partially dispersed into the peripheral cytoplasm as it becomes transformed into a typical somatic vacuole. Oospores swell slightly before the emergence of between one and four germ tubes. These often grow extensively within the oogonium, occasionally infesting and possibly parasitizing neighbouring germlings, before rupturing either the thin-walled pits or basal septa. After emergence most germ tubes continue to grow vegetatively in a sparsely branched fashion, although a few develop terminal sporangia. Oospore germination in this isolate is compared with that described in other oomycete species.