Data from: Fast divergence of Z-linked nucleotide polymorphisms in a genus of lekking birds

Sex chromosomes contribute disproportionately to species boundaries as they diverge faster than autosomes and often have reduced diversity. Their hemizygous nature contributes to faster divergence and reduced diversity, as do some types of selection. In birds, other factors (mating system and bottlenecks) can further decrease the effective population size of Z-linked loci and accelerate divergence (Fast-Z). We assessed Z-linked divergence and effective population sizes for two polygynous sage-grouse species and compared them to estimates from birds with various mating systems. We found lower diversity and higher FST for Z-linked loci than for autosomes, as expected. The πZ/πA ratio was 0.38 in Centrocercus minimus, 0.48 in Centrocercus urophasianus and 0.59 in a diverged, parapatric population of C. urophasianus, a broad range given the mating system among these groups is presumably equivalent. The full data set had unequal males and females across groups, so we compared an equally balanced reduced set of C. minimus and individuals pooled from both C. urophasianus subgroups recovering similar estimates: 0.54 for C. urophasianus and 0.38 for C. minimus. We provide further evidence that NeZ/NeA in birds is often lower than expected under random mating or monogamy. The lower ratio in C. minimus could be a consequence of stronger selection or drift acting on Z loci during speciation, as this species differs strongly from C. urophasianus in sexually selected characters with minimal mitochondrial divergence. As C. minimus also exhibited lower genomic diversity, it is possible that a more severe demographic history may contribute to its lower ratio.性染色体（sex chromosomes）因演化速率快于常染色体（autosomes）且多样性普遍更低，故而对物种界限的塑造具有不成比例的贡献。其半合子特性（hemizygous nature）与部分类型的选择均可加快演化分化并降低多样性水平。在鸟类中，其他因素（交配系统与瓶颈效应）可进一步降低Z连锁（Z-linked）位点的有效种群规模，并加速演化分化（Fast-Z效应）。我们针对两种多配性松鸡物种的Z连锁演化分化与有效种群规模展开评估，并将结果与不同交配系统鸟类的相关估算值进行对比。正如预期，我们发现Z连锁位点的多样性低于常染色体，而遗传分化系数（FST）则更高。Z染色体连锁位点核苷酸多样性与常染色体核苷酸多样性比值（π_Z/π_A）在小艾草松鸡（Centrocercus minimus）中为0.38，在艾草松鸡（Centrocercus urophasianus）中为0.48，在该物种的一个分化邻域种群中为0.59；鉴于这几个类群的交配系统大概率一致，该比值的跨度范围较大。原始数据集的各组雌雄个体数量不均，因此我们选取了经过均衡化处理的缩减数据集：仅纳入小艾草松鸡个体，以及来自两个艾草松鸡亚群的混合个体，重新估算后得到了相似结果：艾草松鸡为0.54，小艾草松鸡为0.38。我们的研究进一步证实，鸟类的Z染色体有效种群规模与常染色体有效种群规模比值（N_eZ/N_eA）通常低于随机交配或一夫一妻制下的预期值。小艾草松鸡的该比值更低，可能是物种形成过程中Z染色体位点受到更强选择或遗传漂变作用的结果——该物种与艾草松鸡在性选择相关性状上差异显著，但线粒体分化程度极低。此外，小艾草松鸡的基因组多样性也更低，因此更严酷的种群历史动态也可能是其比值偏低的成因之一。