All resistance alleles are not equal: the high fitness cost ofsuper-kdrin the absence of insecticide

BACKGROUND Mutations in thevoltage-sensitive sodium channelare an important mechanism of resistance to pyrethroid insecticides. InMusca domestica, common resistance alleles arekdr,super-kdrandkdr-his. The levels of resistance that these alleles confer is known, but the fitness of these alleles relative to each other and to susceptible alleles is unknown. We used crosses from congenic strains ofM. domesticato establish populations with known allele frequencies and then examined the changes in allele and genotype frequencies over 25 generations under laboratory conditions. RESULTS There was a significant fitness cost for thesuper-kdrallele, which decreased from the starting frequency of 0.25 to 0.05 after 25 generations. The fitness of thekdr,kdr-hisand susceptible alleles were similar. The greatest change in genotype frequency was seen for thesuper-kdr/super-kdrgenotype, which was no longer detected after 25 generations. CONCLUSION The fitness cost associated with thesuper-kdrallele is consistent with previous reports and appears to be a factor in helping to restrain high levels of resistance in field populations (thesuper-kdrallele confers higher levels of resistance thankdrorkdr-his). It is known that the relative costs of different alleles are environmentally dependent, but our results also demonstrate that the relative fitness of given alleles depends on which alleles are present in a given population, as previous pairwise comparisons of allele fitness do not exactly match (except forsuper-kdr) the results obtained using this four allele study.

Automated summary

The study found that the super-kdr allele incurs a significant fitness cost compared to other alleles, with its frequency decreasing significantly after 25 generations. In contrast, the fitness of the kdr, kdr-his, and susceptible alleles were similar. The most significant change in genotype frequency was observed for the super-kdr/super-kdr genotype, which was no longer detected after 25 generations.