Data from: Recent speciation and elevated Z-chromosome differentiation between sexually monochromatic and dichromatic species of Australian teals

Sex chromosomes potentially have an important role in speciation and often have elevated differentiation between closely related species. In birds, traits associated with male plumage, female mate preference, and hybrid fitness have been linked to the Z-chromosome (females are heterogametic, ZW). We tested for elevated Z-differentiation between two recently diverged species of Australian ducks, the sexually monochromatic grey teal Anas gracilis and the dichromatic chestnut teal A. castanea. Despite prominent morphological differences, these two species are genetically indistinguishable at both mitochondrial DNA (mean ΦST < 0.0001) and 17 autosomal loci (mean ΦST = 0.0056). However, we detected elevated Z-differentiation (mean ΦST = 0.281) and tentative evidence of an island of differentiation on the Z-chromosome. This elevated differentiation was explained by a high frequency of derived alleles in chestnut teal that were absent in grey teal, which parallels independent evidence for a gain in dichromatism from a monochromatic ancestor. Coalescent estimates of demographic history and simulations indicated that the elevated Z-differentiation was unlikely to be explained by neutral processes, but instead supported a role of divergent selection. We discuss evidence for models of speciation with gene flow versus adaptive divergence in the absence of gene flow and find that both hypotheses are plausible explanations of the data. Overall, these teal have the weakest background differentiation documented to date for a species showing a large Z-effect, and they are an excellent model species for studying speciation genomics and the evolution of sexual dichromatism.性染色体（sex chromosomes）可能在物种形成（speciation）过程中发挥关键作用，且在近缘物种间往往呈现更高的分化水平。在鸟类中，与雄性羽色、雌性配偶选择以及杂交适合度相关的性状均与Z染色体（Z-chromosome）相关联——雌性为异配性别，其染色体组型为ZW。我们以两种近期分化的澳大利亚鸭类为研究对象，即性单色型（sexually monochromatic）的灰水鸭（*Anas gracilis*）与双色型（dichromatic）的栗树鸭（*A. castanea*），旨在探究二者Z染色体上是否存在升高的分化水平。尽管二者形态差异显著，但在线粒体DNA（mitochondrial DNA）与17个常染色体位点（autosomal loci）上均无法通过遗传学手段区分：线粒体DNA的平均ΦST值<0.0001，常染色体位点的平均ΦST值=0.0056。然而，我们检测到Z染色体上存在显著升高的分化水平，平均ΦST值=0.281，并获得了Z染色体上存在分化岛（island of differentiation）的初步证据。这种升高的分化水平可由栗树鸭中高频出现、灰水鸭中缺失的衍生等位基因（derived alleles）解释，这与此前“双色性状由单色祖先演化而来”的独立研究证据相一致。群体历史的溯祖估计（coalescent estimates）与模拟实验结果表明，Z染色体上的升高分化水平不太可能由中性过程（neutral processes）导致，反而支持歧化选择（divergent selection）在其中发挥了关键作用。我们针对“存在基因流（gene flow）的物种形成模型”与“无基因流的适应性分化（adaptive divergence）模型”的相关证据展开讨论，发现两种假说均能合理阐释本研究的数据。总体而言，在已报道的呈现显著Z染色体效应的物种中，这两种水鸭的背景分化水平最低，是研究物种形成基因组学（speciation genomics）以及性双色性状演化的极佳模式物种。