Loss of pyrethroid resistance in newly established laboratory colonies of <i>Aedes aegypti</i>

Background Resistance to pyrethroid insecticides in Aedes aegypti has become widespread after almost two decades of the frequent use of these pesticides to reduce arbovirus transmission. Despite this resistance, pyrethroids continue to be used because they are relatively inexpensive and have low human toxicity. Resistance management has been proposed as a way to retain the use of pyrethroids in natural populations. A key component of resistance management is the assumption that negative fitness is associated with resistance alleles such that resistance alleles will decline in frequency when the insecticides are removed. At least three studies in Ae. aegypti have demonstrated a decrease in pyrethroid resistance once the insecticide has been removed. Methods/Principal findings The present study aims to evaluate variation in the loss of pyrethroid resistance among newly established laboratory populations of Ae. aegypti from Mexico. Eight field collections were maintained for up to eight generations, and we recorded changes in the frequencies of the mutations at the V1,016I locus and at the F1,534C locus in the voltage-gated sodium channel gene (VGSC). I1,016 and C1,534 confer resistance. We also examined resistance ratios (RR) with type 1 and 2 pyrethroids. Conclusions/Significance We demonstrate that, in general, the frequency of the Ae. aegypti pyrethroid-resistance alleles I1,016 and C1,534 decline when they are freed from pyrethroid pressure in the laboratory. However, the pattern of decline is strain dependent. In agreement with earlier studies, the RR was positively correlated with the frequencies of the resistance allele I1,016 and showed significant protection against permethrin, and deltamethrin, whereas F1,534C showed protection against permethrin but not against deltamethrin.研究背景：近二十年来，为降低虫媒病毒（arbovirus）传播，人类频繁使用拟除虫菊酯类杀虫剂（pyrethroid insecticides），导致埃及伊蚊（Aedes aegypti）对该类药剂的抗药性已广泛蔓延。尽管已出现抗药性，拟除虫菊酯类杀虫剂仍被持续使用，原因在于其成本相对低廉且对人类毒性较低。抗性治理（resistance management）被提出作为在自然种群中保留拟除虫菊酯类药剂使用的可行方案。抗性治理的核心假设为：抗性等位基因（resistance allele）会伴随适合度劣势，当移除杀虫剂选择压力后，抗性等位基因的频率会逐渐降低。目前至少已有三项针对埃及伊蚊的研究证实，移除杀虫剂后，其对拟除虫菊酯类药剂的抗药性会出现下降。 材料与方法/主要研究结果：本研究旨在评估来自墨西哥的新建实验室饲养种群埃及伊蚊，其拟除虫菊酯类抗药性消退过程中的差异。本研究对8份野外采集的种群连续饲养至多8代，记录了电压门控钠离子通道基因（voltage-gated sodium channel gene, VGSC）上V1,016I位点与F1,534C位点的突变频率变化。其中I1,016与C1,534突变可赋予抗药性。此外，本研究还检测了针对1型与2型拟除虫菊酯类药剂的抗性比值（resistance ratio, RR）。 结论与意义：本研究证实，总体而言，当实验室种群脱离拟除虫菊酯类药剂的选择压力后，埃及伊蚊的拟除虫菊酯类抗性等位基因I1,016与C1,534的频率会出现下降。但抗性消退的模式因品系而异。与此前研究结果一致，抗性比值与抗性等位基因I1,016的频率呈正相关，且该等位基因可对氯菊酯（permethrin）与溴氰菊酯（deltamethrin）提供抗性保护；而F1,534C突变仅可对氯菊酯提供抗性保护，对溴氰菊酯则无此效果。