A Globally Distributed Insecticide Resistance Allele Confers a Fitness Cost in the Absence of Insecticide in Aedes aegypti (Diptera: Culicidae), the Yellow Fever Mosquito

Abstract

The cosmopolitan mosquito Aedes aegypti is a vector of harmful arboviruses. Pyrethroid insecticides are used to reduce adult populations and prevent the spread of disease. Pyrethroids target the insect voltage-gated sodium channel (VGSC). Collectively, mutations in Vgsc that confer resistance are referred to as knock-down resistance or kdr. There are numerous kdr mutations found in A. aegypti Vgsc, and there is co-occurrence of some mutations. Full-length cDNA sequences have identified nine known kdr (e.g., 1534C) alleles. The 1534C allele is among the most common kdr alleles, but allele frequencies between populations vary considerably. We used the 1534C:RK strain, which has the 1534C (kdr) allele in the genetic background of the insecticide susceptible Rockefeller (ROCK) strain, and conducted population cage experiments to assess the potential intrinsic fitness cost of the 1534C allele relative to the susceptible allele (F1534) in the ROCK strain. Individuals were genotyped across generations using allele specific PCR. A fitness cost of the 1534C allele was detected across seven generations of mosquitos reared in the absence of insecticide selection pressure. The decrease in allele frequency was not due to drift. Comparison of our results to previous studies suggests that the magnitude of the fitness cost of kdr alleles in the absence of insecticide is disconnected from the level of resistance they confer, and that the fitness costs of different kdr alleles can be variable.

Global, asynchronous partial sweeps at multiple insecticide resistance genes inAedesmosquitoes

“…If so, this would require a considerable fitness cost of the resistant allele like that reported for the F1534C mutation in Ae. aegypti 24 . The partial nature of the GST sweeps is further evidence of fitness costs, with no evidence of fixation in any of the populations containing sweeping haplotypes.…”

“…F1534C and V1016G confer strong resistance to Type I pyrethroid insecticides (e.g., permethrin), while V1016G also resists Type II pyrethroids (e.g., deltamethrin) 3 . The advantages of VSSC mutations appear to be offset by physiological costs 24 , which should favour the wild-type susceptible background when insecticide use is low. This trade-off has been observed in Culex mosquitoes, where local frequencies of resistant and wild-type backgrounds rise and fall in response to seasonal insecticide usage 13 .…”

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

“…All replicates were run at the same time. Mosquitoes were reared as previously described [21]. Approximately 800 pupae from the larval containers were placed into cages for each following generation.…”

“…Differences between allele and genotype frequencies across generations were tested using linear mixed models and checked for significance from F-values generated from ANOVA in R as previously described [21].…”

“…Deviation from Hardy-Weinberg equilibrium (HWE) within generations was assessed using a chi-square test (χ2) to compare the observed genotype counts relative to the expected genotype counts calculated using the allele frequencies of the same generation [21]. The level of significance for statistical analyses was p < 0.05.…”