AN ALTERNATIVE POSSIBLE ORIGIN OF THE A2 MATING-TYPE OF PHYTOPHTHORA-INFESTANS OUTSIDE MEXICO

Progeny derived from selfed oospores of an Al (isolate 533) or A2 (isolate 550) field isolate of Phytophthora infestans induced by mating-type-specific hormones consisted of both the A1 and A2 mating types. Oospore progeny produced by selfing of the hybrid of these field isolates also contained the self-fertile A1A2 type in addition to the A1 and A2 types. Selfed-oospore progeny produced by six A1 cultures in S-1 of isolates 533 and 550 consisted of the A1, A2, and A1A2 types; of the Al and A2 types; or of the Al type only. One of the selfed progeny produced by A2 cultures in S-1 of isolate 533 consisted of the A1 and A2 types, while the other consisted of the A2 type only. Results from this study suggest that some individuals of the A2 mating type of P. infestans existing outside Mexico may have originated from selfed oospores produced by the A1 mating type after arriving at the present host countries.