CLONAL DIVERSITY AND GENETIC DIFFERENTIATION OF PHYTOPHTHORA-INFESTANS POPULATIONS IN NORTHERN AND CENTRAL MEXICO

Two Phytophthora infestans populations in northern Mexico (northwestern and northeastern) were analyzed for genetic variation by use of mating type, two allozyme loci, and two DNA fingerprinting probes, and were compared to two populations from central Mexico. The genetic structure of the populations varied widely, even though both mating types occurred in all locations sampled. There was very little genetic diversity in northwestern Mexico; four genotypes were detected among 88 isolates tested, and only two of these were common. All A2 isolates in this population appeared to represent a single clone and were exclusively on potatoes, whereas the A1 isolates were primarily on tomatoes. As the A2 mating type was not known previously in northwestern Mexico, this clone probably represents a recent expansion in its range. In contrast to the low diversity in northwestern Mexico, isolates from northeastern and central Mexico were very diverse. In central Mexico, almost every isolate had a unique genotype, consistent with the hypothesis that sexual reproduction is frequent in this area. There was a moderate degree of genetic differentiation among populations from all four locations (Nei's G(ST) = 0. 12 for allozymes and 0. 14 for DNA), indicating some restriction on gene flow. Although the DNA fingerprinting probes provided a higher degree of resolution, allozymes were surprisingly robust for estimating genetic diversity in many P. infestans populations. A previously unreported allele at the Gpi locus, 111, was found in northwestern and northeastern Mexico, but not in the samples from central Mexico. This allele occurred most often in a five-banded allozyme phenotype that was shown to be attributable to three alleles on different chromosomes in the same individual and, thus, provides the first genetic evidence for elevated ploidy in P. infestans.