All resistance alleles are not equal: the high fitness cost of super‐kdr in the absence of insecticide

BACKGROUND Mutations in the voltage‐sensitive sodium channel are an important mechanism of resistance to pyrethroid insecticides. In Musca domestica , common resistance alleles are kdr , super‐kdr and kdr‐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 of M. domestica to 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 the super‐kdr allele, which decreased from the starting frequency of 0.25 to 0.05 after 25 generations. The fitness of the kdr , kdr‐his and susceptible alleles were similar. The greatest change in genotype frequency was seen for the super‐kdr / super‐kdr genotype, which was no longer detected after 25 generations. CONCLUSION The fitness cost associated with the super‐kdr allele is consistent with previous reports and appears to be a factor in helping to restrain high levels of resistance in field populations (the super‐kdr allele confers higher levels of resistance than kdr or kdr‐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 for super‐kdr ) the results obtained using this four allele study. © 2020 Society of Chemical Industry