Fitness costs in the presence and absence of insecticide use explains abundance of two common Aedes aegypti kdr resistance alleles found in the Americas.

Aedes aegypti is the vector of viruses such as chikungunya, dengue, yellow fever and Zika that have a critical impact on human health. Control of adult mosquitoes is widely done using pyrethroids, but resistance has reduced the effectiveness of this class of insecticides. Resistance to pyrethroids in mosquitoes is commonly due to mutations in the voltage-gated sodium channel (Vgsc) gene (these mutations are known as knockdown resistance, kdr). In the Americas and the Caribbean, the most common kdr alleles are 410L+1016I+1534C and 1534C. In this study, we conducted a population cage experiment to evaluate changes in the allele and genotype frequencies of the 410L+1016I+1534C allele by crossing two congenic strains; one carrying the 410L+1016I+1534C and another with the 1534C allele. Changes in allele frequencies were measured over 10 generations in the absence of insecticide exposure. We also applied one cycle of selection with deltamethrin at F₉ to evaluate the changes in allele and genotype frequencies. Our findings indicate that fitness costs were higher with the 410L+1016I+1534C allele, relative to the 1534C allele, in the absence of deltamethrin exposure, but that the 410L+1016I+1534C allele provides a stronger advantage when exposed to deltamethrin relative to the 1534C allele. Changes in genotype frequencies were not in Hardy-Weinberg equilibrium and could not be explained by drift. Our results suggest the diametrically opposed fitness costs in the presence and absence of insecticides is a reason for the variations in frequencies between the 410L+1016I+1534C and 1534C alleles in field populations. Author summary: Insecticide resistance limits our ability to control vectors of human diseases. Resistance to widely used pyrethroid insecticides can occur by mutations in the voltage gated sodium channel (Vgsc) and alleles with these mutations are collectively known as knockdown resistance (kdr). The frequency of resistance alleles is driven by selection with pyrethroid insecticides, but kdr alleles decrease in frequency in the absence of insecticide. The relative fitness of different kdr alleles to each other is largely unknown. We show that the 1534C allele is favored in the absence of pyrethroid, but that the 410L+1016I+1534C allele is favored when deltamethrin selection occurs. These results help to explain the relative frequency of these alleles that have been detected in field collections. [ABSTRACT FROM AUTHOR]