Ultrastructural changes during zoospore formation in phytophthora parasitica.

Abstract

A B S T R A C T Cleavage of the multinucleate sporangial cytoplasm begiins by a rearrangement and subsequent fusion of randomly dispersed cleavage vesicles. The vesicles line up in planes equidistant from neighboring nuclei and along the sporangial wall. In addition, they contribute to the enlargement of the central vacuole. Fusion of these vesicles with themselves and with the central vacuole cleaves the cytoplasm into uninucleate zoospores, each with two flagella. The sporangial wall consists of two layers, an outer thin one which is continuous over the plug of the discharge pore and an inner thick one which tapers off near the plug. The plug consists of fibrillar material and is ejected upon release of the zoospores. A plug-like structure separating the forming sporangium from the hypha has a homogeneous matrix pervaded with an anastomosing network of fine electron-dense chaiiiels. In additioni, glycogen-like granules occur within mitochondria and paired structures are interpreted as procentrioles. ZOOSPORE PRODUCTION inl the asexual sporaingia of the fungus Phytophthora parasitica Dast. from papaya deserves attention for at least two reasonis. First, it provides a model for the cleavage of a multinucleate cytoplasm into uninucleate portions, a process which is accompanied by formation of a new cell membrane around each resultant zoospore. Secoind, zoospore production represents a stage in the life cycle which is vulnerable to enviroinmental influences and might provide a key for future control of this inotorious plant pathogen (Waterhouse, 1962).