GENETIC DIVERSITY WITHIN AND AMONG RACES AND VEGETATIVE COMPATIBILITY GROUPS OF FUSARIUM-OXYSPORUM F-SP LYCOPERSICI AS DETERMINED BY ISOZYME ANALYSIS

A worldwide collection of 111 isolates of Fusarium oxysporum f. sp. lycopersici from tomato was examined for isozyme polymorphisms. Of 15 enzymes assayed, eight and seven revealed single and multiple bands, respectively, for a total of 30 putative loci. Twenty-two of the 30 loci exhibited complete homogeneity among ail isolates surveyed. Eight loci were variable and revealed two to five putative alleles per locus. Thirty-four electrophoretic phenotypes (EPs) were identified. EPs 1 and 2, with 49 and 13 members, respectively, included 53% of all the isolates. Each of nine EPs contained isolates of different races, vegetative compatibility groups (VCGs), or formae speciales, Thus, a large number of isolates from different races, VCGs, geographic origins, or even formae speciales were electrophoretically identical. Similar patterns were revealed when data were analyzed with simple matching coefficients of similarity, cluster analysis, or principal components analysis. Estimates of genetic similarity among isolates of F. o. lycopersici within a VCG were greater than that of isolates between VCGs. Some VCGs were more similar to each other or to members of other formae speciales than to another VCG within F. o. lycopersici. The majority of EPs that contained isolates from single-member VCGs (VCG 003-) clustered with the multiple-member VCGs. providing evidence for their recent development from the multiple-member VCGs. In general, the distribution of isolates correlated with VCG rather than race, geographic origin, or formae speciales. These results suggest that parasexual recombination does not occur frequently in nature; however, in combination with strong host selection pressures, parasexual recombination could function to introgress genes for altered host specificity. In addition, we conclude that forma specialis lycopersici arose from at least two progenitor populations.