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ABSTRACT
Light and electron microscopy of sexual reproduction in the endoparasite Olpidiopsis varians is discussed and compared with existing accounts of other oomycetous fungi. Thalli that develop sexually are smaller than thalli that develop apogamously. Oogonial thalli are attended by adjacent unicellular antheridial thalli and surrounded by a reticulate network of electron-opaque wall ornamentation. Enzymatic digestion revealed that the wall ornamentation is partially proteinaceous. Nuclear divisions, interpreted as meiotic, occur in the antheridium and oogonium prior to sexual fusion. Meiotic nuclei possess axial filaments during prophase I and a subsequent metaphase plate. The products of meiosis are four closely associated nuclei surrounding a dictyosome-ER complex. Some of the nuclei in sexually reproducing thalli degenerate during meiosis. Peripheral vacuoles within the antheridium expand and compress the cytoplasm while a pore develops between the antheridium and oogonium. The multinucleate antheridial cytoplasm enters the oogonium through the pore and retains its integrity for an undetermined period of time. A plug of wall material fills the antheridial/oogonial pore after sexual fusion. It is proposed that O. varians possesses a diploid life cycle with gametic meiosis.