Experimental swap of Anopheles gambiae's assortative mating preferences demonstrates key role of X-chromosome divergence island in incipient sympatric speciation.

Although many theoretical models of sympatric speciation propose that genes responsible for assortative mating amongst incipient species should be associated with genomic regions protected from recombination, there are few data to support this theory. The malaria mosquito, Anopheles gambiae, is known for its sympatric cryptic species maintained by pre-mating reproductive isolation and its putative genomic islands of speciation, and is therefore an ideal model system for studying the genomic signature associated with incipient sympatric speciation. Here we selectively introgressed the island of divergence located in the pericentric region of the X chromosome of An. gambiae s.s. into its sister taxon An. coluzzii through 5 generations of backcrossing followed by two generations of crosses within the introgressed strains that resulted in An. coluzzii-like recombinant strains fixed for the M and S marker in the X chromosome island. The mating preference of recombinant strains was then tested by giving virgin recombinant individuals a choice of mates with X-islands matching and non-matching their own island type. We show through genetic analyses of transferred sperm that recombinant females consistently mated with matching island-type males thereby associating assortative mating genes with the X-island of divergence. Furthermore, full-genome sequencing confirmed that protein-coding differences between recombinant strains were limited to the experimentally swapped pericentromeric region. Finally, targeted-genome comparisons showed that a number of these unique differences were conserved in sympatric field populations, thereby revealing candidate speciation genes. The functional demonstration of a close association between speciation genes and the X-island of differentiation lends unprecedented support to island-of-speciation models of sympatric speciation facilitated by pericentric recombination suppression.

尽管诸多同域物种形成（sympatric speciation）的理论模型提出，初期物种间负责选型交配（assortative mating）的基因应与重组抑制的基因组区域相关联，但支持该理论的实证数据却寥寥无几。疟蚊（malaria mosquito）中的冈比亚按蚊（Anopheles gambiae）以其通过交配前生殖隔离维持的同域隐蔽物种，以及推测存在的物种形成基因组岛而闻名，因此是研究与初期同域物种形成相关的基因组特征的理想模型系统。本研究通过5代回交，将狭义冈比亚按蚊（An. gambiae s.s.）X染色体着丝粒区域的分化岛选择性导入其姊妹类群科卢齐按蚊（An. coluzzii）中；随后对导入株系进行2代组内杂交，获得了在X染色体分化岛区域固定有M和S标记（M and S marker）的类科卢齐按蚊重组株系。随后，我们为未交配的重组个体提供带有匹配/不匹配自身X岛类型的配偶选择机会，以此检测重组株系的交配偏好。通过对转移精子（transferred sperm）的遗传分析，我们发现重组雌蚊始终与匹配X岛类型的雄蚊交配，从而将选型交配基因与X染色体分化岛关联起来。此外，全基因组测序（full-genome sequencing）证实，重组株系间的蛋白质编码区差异仅局限于实验中互换的着丝粒区域。最后，靶向基因组比对（targeted-genome comparisons）显示，这些独特差异中有不少在同域野外种群（sympatric field populations）中得以保守，由此揭示了候选物种形成基因（candidate speciation genes）。物种形成基因与分化X岛之间紧密关联的功能验证，为依托着丝粒区域重组抑制（pericentric recombination suppression）实现的同域物种形成的物种形成基因组岛模型，提供了前所未有的实证支持。