IMMUNOCYTOCHEMICAL COMPARISON OF PERIPHERAL VESICLES IN ZOOSPORES OF PHYTOPHTHORA AND PYTHIUM SPECIES

An ultrastructural and immunocytochemical comparison has been made of vesicles in the peripheral cytoplasm of zoospores of Phytophthora cinn amomi and species of the related Oomycete genus, Pythium. Our results give evidence of three morphologically and immunologically distinct ve sicle populations in Pythium aphanidermatum and Py. butleri. Large per ipheral vesicles can be recognized by their size and morphology, and b y labeling with monoclonal antibody, Cpa-2 raised against a P. cinnamo mi antigen. They occur predominantly on the dorsal surface of the zoos pores and are retained within the cell during encystment. After encyst ment, the large peripheral vesicles move away from the plasma membrane and become distributed throughout the cyst cytoplasm, in a manner sim ilar to that observed in P. cinnamomi. There are also small vesicles i n 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 t he ridges of the groove on the ventral surface of the zoospores, and t heir contents are secreted during encystment. The secreted material re mains localized on the cyst surface and marks the site of germ tube em ergence, as it does in P. cinnamomi. Other small peripheral vesicles o ccur on the dorsal surface of the Pythium zoospores. These vesicles ar e not labeled by any of the three antibodies tested but disappear duri ng encystment, suggesting that their contents are secreted. Our result s give strong support to the idea that three types of peripheral vesic les are a common feature of zoospores throughout the Peronosporales, a nd that they have similar fates during encystment.