Genetic Differentiation Accompanying Range Expansion by the Southwestern Corn Borer (Lepidoptera: Pyralidae)

Abstract

Five populations of Diatraea grandiosella Dyar were characterized with regard to allele frequencies at five polymorphic allozyme loci, as well as a restriction fragment length polymorphism located in the mitochondrial genome, to quantify genetic differentiation that accompanied a range expansion. Comparison of three populations from localities in the central United States that have been infested since 1930 revealed relatively small but statistically significant allele frequency differences at three of five allozyme loci. Allele frequencies obtained by pooling individuals from these populations were then compared with those from a southern Arizona location close to the putative source of the expansion. Statistically significant heterogeneity was found at three of five loci, the magnitude of those differences being somewhat greater than those from populations within the central United States. Finally, allele frequencies that resulted from pooling individuals from all four U.S. populations were significantly different from those found in a collection from southern Mexico at all five allozyme loci as well as the mitochondrial DNA polymorphism. Clustering of the coancestiy coefficients calculated between pairs of populations resulted in a hierarchy of genetic distance consistent with the putative historical geography of the species. The overall among-population component of genetic variation estimated from the allozyme loci, 0.33, was considerably greater than seen previously in studies of other lepidopteran pests.