Data from: 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 demonstrate that hybrids are fit and responsible for extensive introgression. Nonetheless, we show that strong divergent selection at a subset of loci combined with highly asymmetric introgression has enabled M and S to remain genetically differentiated despite extensive gene flow. We propose that the extent of reproductive isolation between M and S varies across West Africa resulting in a “geographic mosaic of reproductive isolation”; a finding which adds further complexity to our understanding of divergence in this taxon and which has considerable implications for transgenic control strategies.

关于遗传分化（genetic divergence）可在基因流（gene flow）存在的情况下产生并/或得以维持的观点，自首次提出以来便颇具争议。部分具有生殖隔离（reproductive isolation）的类群被视为提供了独特契机，用以探究在基因流存在时维持分化的内在机制。非洲疟疾媒介狭义冈比亚按蚊（Anopheles gambiae s.s.）普遍被认为包含两种同域型，多数研究者认为其代表了初期物种（incipient species），即M与S分子型（molecular forms）。然而，学界围绕M与S之间的生殖隔离程度存在诸多争议：一种观点认为，分化在基因流存在的情况下产生并得以维持；另一种观点则提出，物种形成过程更为进阶，且由于杂交后代适合度（fitness）较低，实际基因流极少。这些假说难以验证，因为杂交后代通常极为罕见（占比<1%）。本研究对高杂交率区域的样本进行了评估，结果表明杂交后代具备适合度，且是广泛基因渐渗（introgression）的诱因。尽管如此，我们的研究显示，部分位点所受的强烈歧化选择（divergent selection），结合高度不对称的基因渐渗，使得M与S尽管存在广泛基因流，仍能维持遗传分化。我们提出，M与S之间的生殖隔离程度在西非各地存在差异，进而形成了‘生殖隔离地理镶嵌格局’；这一发现进一步增加了我们对该类群分化机制的理解复杂度，同时对转基因防控策略（transgenic control strategies）具有重要启示意义。