Genetic Differentiation of Populations of the Southwestern Corn Borer (Lepidoptera: Pyralidae) from the United States and Mexico

Southwestern corn borers, Diatraea grandiosella (Dyar), obtained from seven localities in the United States and from Morelos, Mexico, were subjected to starch and cellulose acetate gel electrophoresis and assayed for 14 enzymes encoded by 17 putative gene loci. Thirteen of the loci expressed no variation. Each of the four polymorphic loci displayed statistically significant site-to-site variation in allele frequencies. Most of the geographic variation occurred between the grouped U.S. samples and the Mexican sample. Particularly notable was a fixed allelic difference between the Mexican sample and the U.S. samples at the mannose phosphate isomerase locus. The geographic variance in allele frequencies was quantified using Wright's Fst measure of population structure. Combined across the four polymorphic loci, Fst equaled 0.272. The average genetic identity (I) between the Mexican sample and each of the U.S. samples was 0.92 when based on all 17 loci. This finding suggests that considerable genetic differentiation has occurred between moths in the United States and a Mexican form and is consistent with previous documentation of physiological and morphological differences between the two groups.