CONTINUING VARIATION IN SUCCESSIVE ASEXUAL GENERATIONS OF PHYTOPHTHORA BOEHMERIAE FOLLOWING SEXUAL REPRODUCTION

The inheritance of colony morphology and growth face in successive zoospore generahons of Phytophthora boehmeriae Sawada following sexual reproduchonwas inveshgated in this paper. The wild-type isolate JS-5 of this species obtained from cotton boil rot hssue in Jiapsu was used as test material. Progeny were devided into parental type A, types B, C and D for those different from type A but with a dishnct uniform pattern, and type M for those with the overlaPPing appearance of tWo different types. The parental isolate JS-5 showed the same colony type as type A and displayed lithe variahon in growth rate in four successive zoospore generahons. However, cultures derived from selfed oosPOres in S. progeny of the same isolate consisted of type A, B, C, D and M, and the oospore cultures showed considerable valuation in growth rate. Of the eighteen selfed oospore cultures tested further,tWelve displayed more than one colony type and showed obvious variahon in growth rate in their zoospore progeny. From the tWelve cultures, tWo M type oospore cultures,JS-5-1 and JS-5 -23, were selected to study the inheritance and variation of the two characters in subsequent progeny of asexual lines of monospores further. The colony mor'Phohology and growth rate of isolate JS-5-1 showed obviously conhnuing vanahon in over three successive zoospore generations. Similar vallahon in zoospore progeny following selfing was observed in isolate JS-5-23. The above resultS suggested that except for causing gene recombination, the sexual reproduchon induced a new mechanism which may engaged in regulating and controlling the inheritance and variahon of the characters. Futhermore, it was observed that over 99% of the zoosPOres produced by isolate JS-5 and itS progeny contained only one nucleus in the cell, which indicated the above mechanism was not associated with heterokaryosis. Thephenomenon of conhnuing variahon in asexual generations induced by sexual reproduction may play an imper'tant role in genehc diversity of biological characters of P. boehmeriae.