VIEW ARTICLE

Genetics

Population Differentiation in the Chestnut Blight Fungus, Cryphonectria parasitica, in Eastern North America. Michael G. Milgroom, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; Susanne E. Lipari, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 85:155-160. Accepted for publication 11 October 1994. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-155.

Isolates of Cryphonectria parasitica were sampled from American chestnut trees in 13 locations in eastern North America to study genetic differentiation and gene flow among subpopulations of this pathogen. Using data from six unlinked restriction fragment length polymorphism loci, we found significant differences in allele frequencies at all loci among the 13 subpopulations. Thirty-one percent of the total gene diversity was attributed to differences among subpopulations (G_ST = 0.31). Genetic differentiation was examined for two subsets of data: one from between two subpopulations in Michigan, which were outside the natural range of the host, and the other from among nine subpopulations from ecologically similar sites (disturbed sites resulting from clearcutting and burning were eliminated). There were significant differences in allele frequencies at two loci between the two Michigan subpopulations, even though these sites were only 16 km apart. Differentiation was high between these subpopulations because two loci were fixed or nearly fixed for different alleles (G_ST = 0.81). There was also differentiation among the nine subpopulations from ecologically similar sites (G_ST = 0.20); significant differences in allele frequencies were found at five of the six loci. We attribute this level of differentiation to restricted gene flow among subpopulations. We did not, however, estimate gene flow quantitatively from these data because populations did not appear to be at equilibrium. There was no correlation between the estimated average number of migrants per generation between each pair of nine subpopulations and geographic distance. This correlation would be expected to be negative if populations were at equilibrium and gene flow was restricted to short distances (isolation by distance). The lack of correlation was interpreted as non-equilibrium conditions; therefore, gene flow would be overestimated.