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 polymorphism s (RFLPs). A total of 791 isolates were sampled from four regions, Chi na, Japan, North America and Europe, and assayed for alleles at eight RFLP loci. Alleles at all eight loci segregated in simple Mendelian ra tios, and most loci were unlinked. Gene diversity was decomposed hiera rchically: 56% of gene diversity was attributable to diversity within subpopulations, compared to 7% among subpopulations within regions, an d 37% among regions. Subpopulations of C. parasitica in China have dis tinctly different allele frequencies from subpopulations in other regi ons, including Japan. DNA fingerprint genotypes in China were also sig nificantly different from those in the other regions. There was an ave rage of 3.2 restriction fragments hybridizing to a DNA fingerprinting probe in Chinese isolates from 11 subpopulations compared to 8.6 in is olates from Japan; the exception was in one subpopulation in northeast ern China where there was an average of 11.1 fragments per isolate. No rth American and European subpopulations were similar to each other, a nd more similar to Japan than to China, for both RFLP allele frequenci es and DNA fingerprints. The results suggest that C. parasitica was in troduced into North America from Japan, not China. The origin of C. pa rasitica for European subpopulations cannot be determined from these r esults, but eastern China was not a likely source. Analysis of populat ion structure within China showed moderate differentiation, with 11% o f gene diversity attributable to differences among subpopulations (G(S T) = O.11). Pairwise estimates of gene flow between subpopulations wer e negatively correlated to geographic distances between subpopulations in China. This result suggests that Chinese populations are in equili brium, and that restricted gene flow and genetic drift shape these pop ulations.