Genetic studies of metabolic resistance to insecticides in field and laboratory populations of house flies

Abstract

Metabolic resistance (R) controlled by a major gene on chromosome II in house flies is maintained in a heterozygous (R/S) condition in a laboratory strain called R-Propoxur-car. After 120 generations of intermittent selection with insecticide, R-Propoxur-car continued to produce two types of flies: normal wing resistant flies and stubby wing (stw) susceptible flies. Detailed genetic analyses revealed that the strain maintained stw and R in a heterozygous condition (R +/S stw) and that there was no recombination between these loci. The lack of recombination indicated that a crossover suppressor such as an inversion was acting to keep the heterozygous condition fixed. In order to test for crossover suppression in field flies, four natural populations of house flies were sampled and isofemale lines were used to prepare recombination maps of chromosome II. In mapping studies R segregated as a single Mendelian gene, but the map distances both between and within the four colonies were highly variable. The resistant lines showed reduced map distances compared to the susceptible lines indicating that the location of the gene had changed, perhaps through an inversion. When flies from natural populations were crossed with laboratory strains for mapping studies, variability in phenotype resulted. Significantly fewer females produced F1 progeny when outcrossed than when sibmated. Wing mutants, mosaics, homeotic mutants, and unstable eye color mutants occurred at high frequency in the F1, F2, and testcross progeny. These mutations occurred in patterns and frequencies similar to systems, such as hybrid dysgensis in Drosophila melanogaster, in which transposable elements have been induced to move.