Genetic variation within and between southern Ontario populations of Sclerotinia homoeocarpa

Restriction fragment length polymorphisms (RFLP) of the intergenic spacer r egion (IGS) of rDNA and random amplified polymorphic DNA (RAPD) markers wer e used to survey genetic variability among 181 isolates of Sclerotinia homo eocarpa from Ontario and 10 isolates from Japan. RAPD and IGS-RFLP analyses revealed polymorphisms within and between populations of S. homoeocarpa, d istinguishing 151 genotypes. Both types of markers gave similar results in phenetic analysis of genetic distances between populations. Cluster analysi s showed that Japanese isolates of S. homoeocarpa were genetically distinct from Ontario isolates, demonstrating significant intraspecific differentia tion. An average genetic similarity of 0.66 was found between Japanese isol ates. Among Ontario isolates, average genetic similarity was 0.86, and geno typic diversity analysis showed that 49.3% of the total genetic variation o bserved within Ontario populations occurred among individuals within popula tions compared to 50.7% between populations. Gametic linkage disequilibrium analysis within Ontario populations revealed an average 15.6% significant nonrandom associations between putative RAPD loci, and that half of the pop ulations showed signs of significant linkage disequilibrium. These results suggest that both clonal propagation and recombination events occurred in l ocal populations of S. homoeocarpa. The high level of genetic similarity be tween populations and the low levels of intraspecific genetic variation may reflect a small founding population fur southern Ontario isolates of S. ho moeocarpa.