Isozyme characterization of sexual and asexual Phytophthora infestans populations

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Abstract
We assayed mycelium of Phytophthora infestans for activity of 38 enzymes using starch gel electrophoresis. Activity was detected for 17 enzymes and 24 loci were resolved. The isozyme diversity found in P. Intestans sampled from an asexual population (representing the United States, Canada, and Europe) was compared to that found in isolates from Mexico, where the sexual stage of the fungus exists. Both populations were monomorphic and identical at 11enzyme loci. We obtained information for only 4 of 13 polymorphic loci due to inadequate resolution of the other 9 loci. The sexual and asexual populations were polymorphic at the Gpi-1 (glucosephosphate isomerase) and Pep (peptidase) loci, while the sexual population alone was polymorphic at the Me (mallc enzyme) and Xdh (xanthine dehydrogenase) loci. Observed banding patterns were consistent with the hypothesis that P. Infestans has a diplold vegetative stage. Fifteen genotypic classes existed in the sexual population, while four classes existed in the asexual population. At the Gpi-1 and Pep loci, genotypic distributions in the sexual population were consistent with Hardy-Weinberg equilibrium, while those in the asexual population were not. This suggests the occurrence of random mating in the Mexican population of P. infestans.

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