Assessing the Effects of <i>Aedes aegypti kdr</i> Mutations on Pyrethroid Resistance and Its Fitness Cost

<div><p>Pyrethroids are the most used insecticide class worldwide. They target the voltage gated sodium channel (Na_V), inducing the knockdown effect. In <i>Aedes aegypti</i>, the main dengue vector, the AaNa_V substitutions Val1016Ile and Phe1534Cys are the most important knockdown resistance (<i>kdr</i>) mutations. We evaluated the fitness cost of these <i>kdr</i> mutations related to distinct aspects of development and reproduction, in the absence of any other major resistance mechanism. To accomplish this, we initially set up 68 crosses with mosquitoes from a natural population. Allele-specific PCR revealed that one couple, the one originating the CIT-32 strain, had both parents homozygous for both <i>kdr</i> mutations. However, this pyrethroid resistant strain also presented high levels of detoxifying enzymes, which synergistically account for resistance, as revealed by biological and biochemical assays. Therefore, we carried out backcrosses between CIT-32 and Rockefeller (an insecticide susceptible strain) for eight generations in order to bring the <i>kdr</i> mutation into a susceptible genetic background. This new strain, named Rock-kdr, was highly resistant to pyrethroid and presented reduced alteration of detoxifying activity. Fitness of the Rock-kdr was then evaluated in comparison with Rockefeller. In this strain, larval development took longer, adults had an increased locomotor activity, fewer females laid eggs, and produced a lower number of eggs. Under an inter-strain competition scenario, the Rock-kdr larvae developed even slower. Moreover, when Rockefeller and Rock-kdr were reared together in population cage experiments during 15 generations in absence of insecticide, the mutant allele decreased in frequency. These results strongly suggest that the <i>Ae. aegypti kdr</i> mutations have a high fitness cost. Therefore, enhanced surveillance for resistance should be priority in localities where the <i>kdr</i> mutation is found before new adaptive alleles can be selected for diminishing the <i>kdr</i> deleterious effects.</p>