Intercontinental population structure of the chestnut blight fungus, Cryphonectria parasitica

The population structure of the chestnut blight fungus, Cryphonectria parasitica was analyzed using restriction fragment length polymorphisms (RFLPs). A total of 791 isolates were sampled from four regions, China, Japan, North America and Europe, and assayed for alleles at eight RFLP loci. Alleles at all eight loci segregated in simple Mendelian ratios, and most loci were unlinked. Gene diversity was decomposed hierarchically: 56% of gene diversity was attributable to diversity within subpopulations, compared to 7% among subpopulations within regions, and 37% among regions. Subpopulations of C. parasitica in China have distinctly different allele frequencies from subpopulations in other regions, including Japan. DNA fingerprint genotypes in China were also significantly different from those in the other regions. There was an average of 3.2 restriction fragments hybridizing to a DNA fingerprinting probe in Chinese isolates from 11 subpopulations compared to 8.6 in isolates from Japan; the exception was in one subpopulation in northeastern China where there was an average of 11.1 fragments per isolate. North American and European subpopulations were similar to each other, and more similar to Japan than to China, for both RFLP allele frequencies and DNA fingerprints. The results suggest that C. parasitica was introduced into North America from Japan, not China. The origin of C. parasitica for European subpopulations cannot be determined from these results, but eastern China was not a likely source. Analysis of population structure within China showed moderate differentiation, with 11% of gene diversity attributable to differences among subpopulations (G(ST) = O.11). Pairwise estimates of gene flow between subpopulations were negatively correlated to geographic distances between subpopulations in China. This result suggests that Chinese populations are in equilibrium, and that restricted gene flow and genetic drift shape these populations.