Electron microscopic study of oospore maturation and germination in an emasculate isolate of Saprolegnia ferax. 4. Nuclear cytology

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Abstract
Changes in nuclear ultrastructure during the formation, maturation, and germination of oospores produced by an agamospermous (emasculate) isolate of Saprolegnia ferax are described. Oogonium nuclei undergo synchronous meiotic divisions which appear identical to those previously reported in antheridiate species. Fusion of two typical gametic nuclei result in the formation of a single, unusually large, restitution nucleus within delimited oospheres. During the activation phase of germination the nuclei undergo three or four successive, near synchronous, mitotic divisions which appear similar to those described in other oomycetes. There was no evidence to support the occurrence of zygotic meiosis. Some nuclei in germinating oospores contain unusual spheroidal and lamellar inclusions, whereas others have abnormally large, irregularly lobed, profiles. Nuclear autolysis was also a common event in germlings. These results are compared with those in other oomycetes and the evidence supporting automictic sexuality discussed.

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