Molecular Evolution of Gustatory Receptors in theAnopheles gambiaeComplex

Abstract

AbstractBackgroundMosquitoes in theAnopheles(An.)gambiaespecies complex are major vectors ofPlasmodium falciparummalaria. One reason for this is the high anthropophily of the constituent speciesAn. coluzzii,An. gambiaesensu stricto, andAn. arabiensis. In contrast, their sister speciesAn. quadriannulatusis highly zoophilic.Anophelesmosquitoes largely rely on chemical cues for host-seeking, which are primarily detected by four chemosensory gene families: olfactory receptors (Ors), ionotropic receptors (Irs), gustatory receptors (Grs), and odorant binding proteins (Obps). Genes from these families that have been implicated in host adaptation show evidence of positive selection in other insect species, including other mosquitoes. As such, we analyzed the molecular evolutionary patterns of the gustatory receptors within theAnopheles gambiaecomplex, with a particular interest in identifyingGrsthat show evidence of positive selection in highly anthropophilic species.ResultsWe identified sixteenGrsthat show evidence of potential positive selection using the McDonald-Kreitman test, including four putative sugar receptors and twoGrswith unknown ligands that are relatively highly expressed in chemosensory organs of eitherAn. coluzziiorAn. quadriannulatus. In addition, we identified twelveGrsthat show evidence of potential purifying selection using the McDonald-Kreitman test, and twelveGrsthat may have experienced a selective sweep using the DH test, including three putative sugar receptors and the carbon dioxide receptorGr24. We also identified both positive and purifying selection in the coastal speciesAn. melas(West Africa) andAn. merus(East Africa).ConclusionsOur results, together with transcriptomic data, identify fourGrsas possible candidates for involvement in the evolution of vertebrate host preference in theAn. gambiaecomplex, as may have occurred in theAn. farauticomplex. They also point to sugar receptors as playing a role in recent adaptation of some of these species. As the vast majority ofGrshave unknown functions and much is still unknown about the role ofGrsin these species, a more complete interpretation of our data necessitates further characterization of these genes.