GENETIC DIFFERENTIATION ACCOMPANYING RANGE EXPANSION BY THE SOUTHWESTERN CORN-BORER (LEPIDOPTERA, PYRALIDAE)

Five populations of Diatraea grandiosella Dyar were characterized with regard to allele frequencies at five polymorphic allozyme loci, as we ll as a restriction fragment length polymorphism located in the mitoch ondrial genome, to quantify genetic differentiation that accompanied a range expansion. Comparison of three populations from localities in t he central United States that have been infested since 1930 revealed r elatively small but statistically significant allele frequency differe nces at three of five allozyme loci. Allele frequencies obtained by po oling individuals from these populations were then compared with those from a southern Arizona location dose to the putative source of the e xpansion. Statistically significant heterogeneity was found at three o f five loci, the magnitude of those differences being someu what great er than those from populations within the central United States. Final ly 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 a s the mitochondrial DNA polymorphism. Clustering of the coancestry coe fficients calculated between pairs of populations resulted in a hierar chy of genetic distance consistent with tile putative historical geogr aphy of the species. The overall among-population component of genetic variation estimated from the allozyme loci, 0.33, was considerably gr eater than seen previously in studies of other lepidopteran pests.