The biological costs and benefits associated with a pyrethroid-resistant genotype of Aedes aegypti

Abstract

Aedes aegypti Linnaeus (Diptera: Culicidae) is an important vector of mosquito-borne viruses, including dengue, Zika and chikungunya. In the case of dengue, approximately half of the world’s population is at risk, and an estimated 100–400 million new infections are reported each year. To control Ae. aegypti populations, mosquito management programmes rely heavily on insecticides, which have been the mainstay of mosquito management programmes for almost 50 years. The most common class of insecticide used to control mosquito vectors of human disease are the pyrethroids. These target the voltage-gated sodium channel, an essential protein that facilitates electrical signalling within the nervous system. Due to the intense selection pressures applied by public health programmes and the incidental exposure of mosquitoes to agricultural and domestic insecticides, pyrethroid-resistant Ae. aegypti populations are now common.Target site resistance, one of the common mechanisms associated with resistance to pyrethroids, occurs most frequently in mosquitoes through non-synonymous mutations in the voltage-gated sodium channel gene. These point mutations, commonly known as knockdown resistance (kdr) mutations, can cause a change in the shape of the voltage-gated sodium channel, which decreases the efficacy with which pyrethroids bind to them. Knockdown resistance mutations in Ae. aegypti populations have been reported from many parts of the world and reflect the increasingly common reports of phenotypic resistance to pyrethroids on a global scale. The geographically widespread distribution of pyrethroid-resistant Ae. aegypti has emphasised the requirement for insecticide resistance management strategies that conserve susceptibility and ensure the availability of effective insecticidal chemistries to control mosquito-borne disease. Insecticide Resistance Management (IRM) strategies rely on reducing selection pressure, which can result in the immigration of susceptible genotypes and, where they exist, allow fitness costs to reduce the frequency of resistance mutations. With regard to the latter, our understanding of the consequences of kdr mutations on the fitness and behaviour of Ae. aegypti, in the absence of pyrethroids, is limited. Furthermore, we do not have a clear understanding of how Ae. aegypti with or without these mutations are affected by the sublethal doses of insecticide that are common in urban and peri-urban habitats.I backcrossed a pyrethroid-resistant strain of Ae. aegypti from Timor-Leste, harbouring the V1016G and S989P kdr mutations, with an insecticide susceptible strain from Queensland, Australia that does not carry kdr mutations. This produced a strain of Ae. aegypti that was homozygous for the V1016G and S989P kdr mutations in an otherwise susceptible genetic background. Using double-digest restriction associated sequencing, I confirmed that >99.9% of the backcrossed strain’s genome originated from the susceptible parental strain. The creation of the backcrossed strain allowed me to compare differences in life-history traits and behaviour between the backcrossed-resistant and susceptible parent strain, in the absence or presence of permethrin, and to directly attribute those distinctions to the effects of the homozygous V1016G/S989P genotype.The results presented within this thesis demonstrate that, in comparison to the susceptible strain, mosquitoes with the V1016G/S989P genotype exhibited longer larval development times, had smaller wing lengths and females had a shorter average lifespan (Chapter II). Investigations into behavioural changes showed a significant reduction in mean wing beat frequency in males and a significant reduction in estimated male mating success, associated with the introgressed kdr genotype (Chapter III). These results suggest specific and significant fitness costs and behavioural changes associated with the double homozygous V1016G/S989P genotype, in the absence of insecticides. These likely consequences of resistance evolution may encourage reversion to susceptibility in the absence of insecticide selection pressures.I also examined the impact of sublethal permethrin exposure on the mosquito fitness parameters of survival, fecundity, fertility, blood-feeding, host location and mating success. For the first time, I demonstrated some expected, negative impacts of sublethal exposure on susceptible Ae. aegypti mosquitoes but also showed that detrimental effects on longevity and host location can occur even in highly resistant mosquitoes (Chapter IV). Overall, I demonstrated that significant differences in fitness exists between mosquitoes with susceptible and resistant genotypes, in the presence of sublethal doses of insecticides. The competitive advantage conferred to resistant mosquito strains in the presence of sublethal doses of permethrin from various sources, questions our ability to create and exploit untreated refugia or mosaics and therefore remove selection pressures and permit resistance-associated fitness costs to manifest.My results focus on the pleiotropic effects of a common kdr genotype on the biology and behaviour of Ae. aegypti in the absence and presence of insecticides. By cataloguing these effects, this thesis provides evidence to inform the introduction of specific IRM measures into vector control programmes and predict their potential impact by cataloguing these effects. My results are discussed in relation to reducing pyrethroid selection pressures and the subsequent impact on the frequency of resistant genotypes by temporarily removing pyrethroid selection pressures as part of IRM and encouraging the consequent recovery or conservation of pyrethroid susceptibility.