Mosquitoes strain used in the experiments
Mosquito strains used in this study consisted of a deltamethrin selected resistant colony and an unselected susceptible colony (hereafter referred to as resistant and susceptible mosquitoes) that were collected from Bungoma in western Kenya. These colonies were selected and maintained at the Centre for Global Health Research, Kenya Medical Research Institute (KEMRI) in Kisumu (Machani et al. , 2020), Western Kenya, under standard rearing conditions of 27 ± 2 °C and relative humidity (RH) of 80 ± 10% °C under a L12: D12 h light: dark cycle. During the process, each colonized strain had three independent lineages that started with 200-250 females at every new generation to limit bottleneck effects. The progeny of F1 wild-caught mosquitoes from the same site were also used to undertake these experiments.
Resistant strain: This colony underwent deltamethrin selection after each generation. The 6th generation used was highly resistant with 20% mortality according to the WHO criteria (WHO, 2016). Resistance in this colony was mainly mediated by cytochrome P450 detoxification enzyme. The two kdr mutations 1014S and 1014F were present and at high frequencies (Machani et al. , 2020).
Susceptible strain: This strain shares the same genetic background with the resistant colony, however, it was reared in the absence of insecticide selection pressure. After 9 generations without selection pressure, the population had almost lost resistance to deltamethrin (92%) and after 13 generations the population showed increased mortality (97.3%). The vgsc1014S was at a high frequency given that the allele was already fixed in the parent population(Machani et al. , 2020). The 14th generation was used in this study.
Wild population : F1 progeny obtained from wild-caught An. gambiae female mosquitoes from the same area where the resistant and susceptible colonies were selected were used for validation. Each female(mother) was identified by PCR as An. gambiae s.s according to the methods of Scott et al.(1993).The wild population had 56% resistance to deltamethrin. The observed resistance is mediated by a mix of metabolic and kdr(Bonizzoni et al. , 2012; Ochomo et al. , 2012; Wanjala et al. , 2015; Machani et al. , 2019; Machani et al. , 2020).