Abstract
Afrotropical malaria vectors Anopheles gambiae and An. coluzzii are sister species in the Gambiae complex diverging ecologically and genetically despite occasional hybridization (<1%). An exception is a region in ‘Far-West’ Africa, ranging from Guinea-Bissau to Senegal, where unusually high hybridization rates (3%-24%) have been recorded. Recent whole-genome analyses of populations from Guinea Bissau and The Gambia highlighted the occurrence of a distinct hybrid taxon having a mosaic genome inherited from the two species. This hybrid zone constitutes a suitable region where to study the genetic architecture of speciation, and identify mechanisms allowing these taxa to maintain their integrity in the face of gene flow.To characterize the genetic and eco-geographical structure of 24 populations along the Casamance river drainage basin in southern Senegal—a region lying inside the hybrid zone and in-between the localities where the hybrid taxon was reported—we carried out two surveys separated by a 5-yr interval, across a 300-km transect spanning a gradient of water salinity. More than 3,000 An. gambiae s.s. from 175 breeding sites characterized for their physico-chemical profile, were genotyped. The hybrid status of each larva was assessed by the the Divergent Islands SNP assay aimed at distinguishing parentals, hybrids, and backcross individuals.Larvae that could be considered as F1 hybrids where rare, indicating that hybridization between parentals is occasional in this region too. Hybrid genotypes on the X chromosome were markedly less than on the 2L and 3L divergence islands. A positive correlation between hybridization and salinity emerged from the distinct distribution of genotypes on the opposite sides of the salinity ecotone generated by an anti-salt dam: less hybridized genotypes prevailed in the freshwater section, while more hybridized genotypes prevailed on the saltwater side. Larvae with mostly An. coluzzii ancestry-informative markers on chromosome X clustered in populations at and just upstream of the ecotone.