Self-fertility in Phytophthora infestans: heterokaryons segregate several phenotypes

Isolates from potato in an isolated garden in North Wares in 1995 consisted of essentially two distinct phenotypes. Phenotypes P1 and P2, of Al and A2 mating type respectively, had distinctive multi-locus (RG57) and telomeric RFLP fingerprints. In addition, about 6% of isolates forming abundant oosp ores were self-fertile and showed a combination of both fingerprints. Among a total of 40 single-hyphal-tip and single-sporangial propagations From tw o self-fertile isolates, 16 were P2 whereas only one propagation was P1: fo ur propagatins were still self-fertile and still carried the molecular mark ers typical of both P1 and P2. Most of the remainder were Ar but otherwise carried both P1 and P2 molecular markers. Uninucleate zoospore lines from t wo of these were also of AI mating type but had segregated molecular marker s typical of P2; a zoospore line from a third segregated both A2 mating typ e and markers typical of P2. The original self-fertile isolates appeared to have heterokaryotic hyphae and sporangia with P1 and P2 nuclei and also nu clei of Ar marine type but PZ markers. Tetraploid nuclei within a heterokar yon, resulting from fusion of PI with P2 nuclei, could have segregated to y ield the latter.