Using intron sequence comparisons in the triose-phosphate isomerase gene to study the divergence of the fall armyworm host strains

The noctuid moth Spodoptera frugiperda (the fall armyworm) is endemic to the Western Hemisphere and appears to be undergoing sympatric speciation to produce two subpopulations that differ in their choice of host plants. The 'rice strain' and 'corn strain' are morphologically indistinguishable, requiring the use of genetic markers for identification. Because fall armyworm is a major pest of corn and several other agricultural crops, characterizing the strains has important economic consequences. In this study, comparisons were made of the intron sequences from the triose-phosphate isomerase (Tpi) gene isolated from 85 fall armyworm specimens collected from two host plants. Sixteen new strainspecific haplotypes based on intron polymorphisms are described that can facilitate the characterization of fall armyworm populations associated with different host plants. Comparisons of genetic diversity within and between the strains provides evidence that the corn strain is undergoing active selection and supports the proposal of directional interstrain mating occurring in the wild. Comparisons of the polymorphisms indicate that each intron undergoes different patterns of mutation that in some cases corresponds to host plant preferences. The results confirm that intron sequence comparisons are an effective approach to study fall armyworm population genetics.