Molecular Population Genetics of Sodium Channel and Juvenile Hormone Esterase Markers in Relation to Pyrethroid Resistance in Heliothis virescens (Lepidoptera: Noctuidae)

Abstract

Pyrethroid insecticide resistance in strains of tobacco budworm, Heliothis virescens (F.), has been linked to a locus hscp , which encodes the target of action of pyrethroids, a voltage-gated sodium channel (α subunit). Using denaturing gradient gel electrophoresis, we examined allelic variation for the DNA marker locus Hpy within this gene, and for an additional marker locus Hejs within a juvenile hormone esterase gene, for population samples from the southern United States in 1990 and 1995. FST, the estimate of genetic variance among populations for Hpy was 0.041 (SE = 0.005), significantly greater than the FST of 0.001 (SE = 0.003) for Hejs , and other estimates reported for allozyme loci in a 1989 collection over a similar geographic range. The greater variance for Hpy suggests that selection had been acting at or near the hscp locus. There was a significant excess of Hpyl, 3 , and 5 homozygotes in the 1990 collection, relative to Hardy–Weinberg expectation. However, there was also excess homozygosity for Hejs alleles, for reasons unknown. Hpy3 , the 3rd most common allele of Hpy , significantly increased after pyrethroid selection on Texas and Louisiana samples of 1990, and on progeny of a Louisiana sample of 1995. This indicates positive linkage disequilibrium between Hpy3 and presumed resistance-conferring allele(s) at another locus. Disequilibrium has persisted from 1990 to 1995. We conclude that one or more mutations elsewhere in the hscp gene confer resistance to pyrethroids in populations of H. virescens .