Abstract
An investigation has been made of selffertile (homothallic) progeny which are frequently encountered in matings of heterothallic Phytophthora species. The pattern of segregation of self-sterility from these homothallic isolates during vegetative growth, asexual and sexual reproduction has been studied in some detail. Heterokaryosis was shown not be the cause of this secondary homothallism in a number of cases investigated since homothallism could be transmitted by single uninucleate zoospores. The homothallic cultures (A1 A2) showed different degrees of stability. One culture, obtained as a very fertile sector from an original A1 A2 gave only A1 A2 phenotypes on repeated single zoospore analysis, whilst two other sectors from the same A1 A2 culture gave in addition to A1 A2, some A2 and a few A1 phenotypes. The A2 types derived from this homothallic were also unstable, giving some A1 A2 and occasional A1 types as well as A2 types in single zoospore cultures. This contrasts with the stability of the parental A2 culture. Self-fertility was transmitted through the oospore to the sexual progeny. Most progeny were A1 A2 but heterothallic A1 and A2 types segregated. It is suggested that the self-fertile condition was due to the presence of an extra chromosome containing the mating type locus, the A1 A2 type being Aaa. A cytological investigation to test this trisomic hypothesis is being reported separately.
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