Genetic variation within and between southern Ontario populations of Sclerotinia homoeocarpa

Restriction fragment length polymorphisms (RFLP) of the intergenic spacer region (IGS) of rDNA and random amplified polymorphic DNA (RAPD) markers were used to survey genetic variability among 181 isolates of Sclerotinia homoeocarpa from Ontario and 10 isolates from Japan. RAPD and IGS-RFLP analyses revealed polymorphisms within and between populations of S. homoeocarpa, distinguishing 151 genotypes. Both types of markers gave similar results in phenetic analysis of genetic distances between populations. Cluster analysis showed that Japanese isolates of S. homoeocarpa were genetically distinct from Ontario isolates, demonstrating significant intraspecific differentiation. An average genetic similarity of 0·66 was found between Japanese isolates. Among Ontario isolates, average genetic similarity was 0·86, and genotypic diversity analysis showed that 49·3% of the total genetic variation observed within Ontario populations occurred among individuals within populations compared to 50·7% between populations. Gametic linkage disequilibrium analysis within Ontario populations revealed an average 15·6% significant nonrandomassociations between putative RAPD loci, and that half of the populations showed signs of significant linkage disequilibrium. These results suggest that both clonal propagation and recombination events occurred in local populations of S. homoeocarpa. The high level of genetic similarity between populations and the low levels of intraspecific genetic variation may reflect a small founding population for southern Ontario isolates of S. homoeocarpa.