Submitted to: Phytopathology
Publication Type: Abstract Only
Publication Acceptance Date: June 1, 1997
Publication Date: N/A
Technical Abstract: Cluster analyses of genotypes of Phytophthora infestans from six locations where the A2 mating type was detected recently were used to explicitly test the mating type change hypothesis for the origin of the A2 outside Mexico. Origin by mating type change predicts that A1s and A2s should occur in near identical genetic backgrounds. However, in all six locations A2s did not cluster with the previously existing A1s and the mating type change hypothesis was rejected. "New" isolates were always very different from "old" isolates. Migration is the only explanation for these results. Early reports of oospores were evaluated for evidence of homothallism. In all cases, oospores were produced only sporadically in old cultures under special conditions. The few oospores produced rarely had antheridia and usually were aborted, so none of the isolates described in early reports could be classified as homothallic. A previous conclusion that A2s were found in Japan during the 1930s also was evaluated and was not supported. There was no evidence for the occurrence of A2 or homothallic isolates of P. infestans in any location prior to their discovery in central Mexico during the 1950s.