Investigation of the influence of a glutathione S-transferase metabolic resistance to pyrethroids/DDT on mating competitiveness in males of the African malaria vector, Anopheles funestus

Background: Metabolic resistance is a serious challenge to current insecticide-based interventions. The extent to which it affects natural populations of mosquitoes including their reproduction ability remains uncharacterised. Here, we investigated the potential impact of the glutathione S-transferase L119F-GSTe2 resistance on the mating competitiveness of male Anopheles funestus, in Cameroon.

Methods: Swarms (using sweep net) and indoor resting (using electric aspirators) collections took place in March, 2018 in Tibati, Cameroon. Firstly, to assess the susceptibility profile of malaria vectors in this locality, WHO tube and cone assays were performed on F1 mosquitoes from indoor collected females. Using a new allele-specific genotyping method, mosquitoes mated and unmated males collected in the swarms was genotyped for the L119F metabolic marker to assess the association between this marker and the mating male competitiveness.

Results: Susceptibility and synergist assays, showed that this population was multiple resistant to pyrethroids, DDT and carbamates likely driven by metabolic resistance mechanisms. Cone assays revealed a reduced efficacy of standard pyrethroid-nets (Olyset and PermaNet 2.0) with low mortality (<25%) whereas synergist PBO-Nets (Olyset Plus and PermaNet 3.0) retained greater efficacy with higher mortality (>80%). The L119F-GSTe2 mutation, conferring pyrethroid/DDT resistance, was detected in this An.funestus population at a frequency of 28.8%. In addition, a total of 15 mating swarms were identified and 21 An. funestus couples were isolated from those swarms using sweep nets. A comparative genotyping of the L119F-GSTe2 mutation between mated and unmated males revealed that heterozygote males 119L/F-RS were less able to mate than homozygote susceptible (OR=7.2, P<0.0001). Surprisingly, heterozygote mosquitoes were also less able to mate than homozygote resistant (OR=4.2, P=0.010) suggesting the presence of a heterozygote disadvantage effect. Overall, mosquitoes bearing the L119-S susceptible allele were significantly more able to mate than those with 119F-R resistant allele (OR=2.1, P=0.03).

Conclusion: This study revealed that An. funestus swarms can be detected and characterized in the field allowing mating swarms to be targeted to implement alternative vector control strategies. Furthermore this study provides preliminary evidences that metabolic resistance potentially exerts a fitness cost on mating competiveness in resistant mosquitoes.