Variable fitness costs for pyrethroid resistance alleles in the house fly, Musca domestica, in the absence of insecticide pressure

Resistance to pyrethroids is caused by mutations in the voltage-sensitive sodium channel (Vssc) and enhanced metabolic detoxification by CYP6D1 in the house fly, Musca domestica. We investigated if there was a fitness cost associated with resistance alleles by performing a competition study with multiple Vssc and CYP6D1 alleles under laboratory conditions in the absence of insecticides. The kdr1 haplotype is significantly favored over the kdr2, super-kdr and susceptible v3 haplotypes. The initial frequencies of kdr2, super-kdr and v3 declined and remained low through the course of the experiment. The v5 and v6 haplotypes did not change from their initial frequency and were considered to be neutral. Genotypes containing either kdr2 or v3 became increasingly rare despite being the most frequent genotypes at the initiation of the experiment. Heterozygotes containing kdr1 and kdr1/kdr1 homozygotes accounted for the majority of genotypes after the F1. The susceptible v5 and v6 haplotypes were mostly found as heterozygotes with kdr1, and any combination of v5 and v6 did not exceed 5% at any generation. This suggests that kdr1 carries a fitness advantage and kdr2, super-kdr and v3 are at a fitness disadvantage under the environmental conditions of this experiment. The frequency of the resistant CYP6D1v1 allele increased over the course of the experiment, but did not deviate significantly from HWE. Thus, there is no fitness cost for the CYP6D1v1 allele under these conditions. These results are compared to previous research on field collected populations, and the impact of the fitness advantage of an insecticide resistance allele on insecticide resistance evolution and management is discussed.

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► Examination of Vssc haplotypes reveals fitness costs not observed when only susceptible vs. resistant alleles are considered.
► Under laboratory conditions the kdr1 haplotype is favored while kdr2, super-kdr, and v3 decline in frequency.
► The kdr1 haplotype increased in frequency from 17% in the F1 to >50% by the F30.