Development and ultrastructure of the marine, parasitic oomycete, Lagenisma coscinodisci (Lagenidiales): sexual reproduction

Fine structural details of antheridia and oogonia, of cytogamy, plasmogamy, and karyogamy, and finally of the development of the oospore are described. Sexually determined zoospores (most probably zoomeiospores) resemble vegetative primary zoospores (zoomitospores). They form gametangia (antheridia or oogonia) which are similar to vegetative primary cysts. An antheridium germinates with a fertilization tube which penetrates the oogonium. As seen in abnormal cell fusions, the tip of the fertilization tube opens only within an oogonium; presumably, its wall is disintegrated by the latter. Before plasmogamy, both plasmalemmata are closely appressed. The fused protoplasts move into a short zygotial hypha which protudes from the oogonium. The thick-walled oospore is formed herein. It includes nearly all of the cytoplasm of the gametangia. Karyogamy takes place within the developing oospore. It is initiated by an interdigitation of microtubules radiating from the centrioles and by a close association of the latter. The observations are compared with those on other developmental stages of Lagenisma and with those on gametangia and oospores of other Oomycetes.