Immunocytochemical comparison of peripheral vesicles in zoospores ofPhytophthoraandPythiumspecies

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An ultrastructural and immunocytochemical comparison has been made of vesicles in the peripheral cytoplasm of zoospores of Phytophthora cinnamomi and species of the related Oomycete genus, Pythium. Our results give evidence of three morphologically and immunologically distinct vesicle populations in Pythium aphanidermatum and Py. butleri. Large peripheral vesicles can be recognized by their size and morphology, and by labeling with monoclonal antibody, Cpa-2 raised against a P cinnamomi antigen. They occur predominantly on the dorsal surface of the zoospores and are retained within the cell during encystment. After encystment, the large peripheral vesicles move away from the plasma membrane and become distributed throughout the cyst cytoplasm, in a manner similar to that observed in P. cinnamomi. There are also small vesicles in the Pythium zoospore cortex. Some are identified as ventral vesicles through their reaction with monoclonal antibody Vsv-1, raised against a P. cinnamomi antigen. The ventral vesicles are concentrated along the ridges of the groove on the ventral surface of the zoospores, and their contents are secreted during encystment. The secreted material remains localized on the cyst surface and marks the site of germ tube emergence, as it does in P. cinnamomi. Other small peripheral vesicles occur on the dorsal surface of the Pythium zoospores. These vesicles are not labeled by any of the three antibodies tested but disappear during encystment, suggesting that their contents are secreted. Our results give strong support to the idea that three types of peripheral vesicles are a common feature of zoospores throughout the Peronosporales, and that they have similar fates during encystment.