Asymmetric introgression between the M and S molecular forms of the malaria vector, Anopheles gambiae, maintains divergence despite extensive hybridisation.

The suggestion that genetic divergence can arise and/or be maintained in the face of gene flow, has been contentious since first proposed. Partially reproductively isolated taxa have been highlighted as offering unique opportunities for identifying the mechanisms underlying the maintenance of divergence with gene flow. The African malaria vector, Anopheles gambiae s.s., is widely regarded as consisting of two sympatric forms, thought by many to represent incipient species, the M and S molecular forms. However, there has been much debate about the extent of reproductive isolation between M and S, with one view positing that divergence may have arisen and is being maintained in the presence of gene flow, and the other proposing a more advanced speciation process with little realized gene flow due to low fitness of hybrids. These hypotheses have been difficult to address because hybrids are typically very rare (<1%). Here, we assess samples from an area of high hybridisation and demonst...