Parallel evolution or purifying selection, not introgression, explains similarity in the pyrethroid detoxification linked GSTE4 of Anopheles gambiae and An. arabiensis

Abstract

Insecticide resistance is a major impediment to the control of vectors and pests of public health importance and is a strongly selected trait capable of rapid spread, sometimes even between closely-related species. Elucidating the mechanisms generating insecticide resistance in mosquito vectors of disease, and understanding the spread of resistance within and between populations and species are vital for the development of robust resistance management strategies. Here we studied the mechanisms of resistance in two sympatric members of the Anopheles gambiae species complex – the major vector of malaria in sub-Saharan Africa – in order to understand how resistance has developed and spread in eastern Uganda, a region with some of the highest levels of malaria. In eastern Uganda, where the mosquitoes Anopheles arabiensis and An. gambiae can be found sympatrically, low levels of hybrids (0.4%) occur, offering a route for introgression of adaptively important variants between species. In independent microarray studies of insecticide resistance, Gste4, an insect-specific glutathione S-transferase, was among the most significantly up-regulated genes in both species. To test the hypothesis of interspecific introgression, we sequenced 2.3kbp encompassing Gste4. Whilst this detailed sequencing ruled out introgression, we detected strong positive selection acting on Gste4. However, these sequences, followed by haplotype-specific qPCR, showed that the apparent up-regulation in An. arabiensis is a result of allelic variation across the microarray probe binding sites which artefactually elevates the gene expression signal. Thus, face-value acceptance of microarray data can be misleading and it is advisable to conduct a more detailed investigation of the causes and nature of such signal. The identification of positive selection acting on this locus led us to functionally express and characterise allelic variants of GSTE4. Although the in vitro data do not support a direct role for GSTE4 in metabolism, they do support a role for this enzyme in insecticide sequestration. Thus, the demonstration of a role for an up-regulated gene in metabolic resistance to insecticides should not be limited to simply whether it can metabolise insecticide; such a strict criterion would argue against the involvement of GSTE4 despite the weight of evidence to the contrary.

Genome‐wide gene expression profiling reveals that cuticle alterations and P450 detoxification are associated with deltamethrin and DDT resistance in Anopheles arabiensis populations from Ethiopia

“…Notably, in addition to epicuticular hydrocarbon enrichment, compositional changes of the cuticle have also been associated with insecticide resistance and several genes have been associated with the phenomenon . Finally, glutathione‐S‐transferases (GSTs) are also important enzymes involved in toxicokinetic resistance mechanisms against pyrethroids . For example, Riveron et al 2014, 2017, showed that allelic variation and higher transcription of GSTe2 confers resistance against permethrin in an An.…”

Cytochrome P450s in Anopheles gambiae (Diptera: Culicidae) and Insecticide Resistance in Africa: A Mini Review

“…It has been well documented that P450-mediated pyrethroid resistant insects strains have higher levels or more efficient enzyme forms of one or more P450s compared to susceptible strains [11][12][13]. Cytochrome P450s are therefore generally associated with the enzymatic metabolism of insecticides [14]. A number of studies have been established on the characteristic features of P450s in higher mammals including man and a model insect Drosophila melanogaster.…”

Influence of GST- and P450-based metabolic resistance to pyrethroids on blood feeding in the major African malaria vectorAnopheles funestus

“…This observation is intriguing as some salivary genes such as D7 family genes were previously reported to be over-expressed in resistant An. funestus mosquito compared to susceptible strain [22,44–47]. The absence of significance of genes differential expression in the present study could be explained by the fact that, our analyses in this study were performed on mosquitoes obtained after reciprocal crosses between two different strain and therefore sharing the same background, while other studies compared insecticide resistant field mosquitoes and susceptible laboratory strains.…”