Department of Biology, Stanford University, Stanford, California, 94305.
Centro de Investigaciones Científicas de las Huastecas "Aguazarca", A.C., Calnali, 43230, Mexico.
Evolution. 2021 Oct;75(10):2524-2539. doi: 10.1111/evo.14337. Epub 2021 Sep 13.
Natural hybridization events provide unique windows into the barriers that keep species apart as well as the consequences of their breakdown. Here, we characterize hybrid populations formed between the northern swordtail fish Xiphophorus cortezi and Xiphophorus birchmanni from collection sites on two rivers. We use simulations and new genetic reference panels to develop sensitive and accurate local ancestry calling in this novel system. Strikingly, we find that hybrid populations on both rivers consist of two genetically distinct subpopulations: a cluster of pure X. birchmanni individuals and one of phenotypically intermediate hybrids that derive ∼85-90% of their genome from X. cortezi. Simulations suggest that initial hybridization occurred ∼150 generations ago at both sites, with little evidence for contemporary gene flow between subpopulations. This population structure is consistent with strong assortative mating between individuals of similar ancestry. The patterns of population structure uncovered here mirror those seen in hybridization between X. birchmanni and its sister species, Xiphophorus malinche, indicating an important role for assortative mating in the evolution of hybrid populations. Future comparisons will provide a window into the shared mechanisms driving the outcomes of hybridization not only among independent hybridization events between the same species but also across distinct species pairs.
自然杂交事件为了解物种之间的隔离障碍以及它们的破坏后果提供了独特的窗口。在这里，我们描述了来自两条河流的采集点的北方剑尾鱼（Xiphophorus cortezi）和 Xiphophorus birchmanni 之间形成的杂交种群。我们使用模拟和新的遗传参考面板来开发在这个新系统中敏感和准确的局部祖先调用。引人注目的是，我们发现两条河流上的杂交种群都由两个具有不同遗传特征的亚种群组成：一群纯 X. birchmanni 个体和一群表型中间的杂种，它们的基因组约有 85-90%来自 X. cortezi。模拟表明，最初的杂交大约发生在两个地点 150 代以前，两个亚种群之间几乎没有当代基因流的证据。这种种群结构与具有相似遗传背景的个体之间的强选择性交配一致。这里发现的种群结构模式与 X. birchmanni 与其姐妹种 Xiphophorus malinche 之间的杂交所看到的模式相似，表明在杂交种群的进化中，选择性交配起着重要作用。未来的比较将为我们提供一个窗口，了解不仅在同一物种之间的独立杂交事件中，而且在不同物种对之间，导致杂交结果的共同机制。
