Data from: Opposing patterns of intraspecific and interspecific differentiation in sex chromosomes and autosomes

Linking intraspecific and interspecific divergence is an important challenge in speciation research. X chromosomes are expected to evolve faster than autosomes and disproportionately contribute to reproductive barriers, and comparing genetic variation on X and autosomal markers within and between species can elucidate evolutionary processes that shape genome variation. We performed RADseq on a 16-population transect of two closely-related Australian cricket species, Teleogryllus commodus and T. oceanicus, covering allopatry and sympatry. This classic study system for sexual selection provides a rare exception to Haldane’s rule, as hybrid females are sterile. We found no evidence of recent introgression, despite the fact that the species co-exist in overlapping habitats in the wild and interbreed in the laboratory. Putative X-linked loci showed greater differentiation between species compared to autosomal loci. However, population differentiation within species was unexpectedly lower on X-linked markers than autosomal markers, and relative X-to-autosomal genetic diversity was inflated above neutral expectations. Populations of both species showed genomic signatures of recent population expansions, but these were not strong enough to account for the inflated X/A diversity. Instead, most of the excess polymorphism on the X could better be explained by sex-biased processes that increase the relative effective population size of the X, such as interspecific variation in the strength of sexual selection among males. Taken together, the opposing patterns of diversity and differentiation at X versus autosomal loci implicate a greater role for sex-linked genes in maintaining species boundaries in this system.

解析种内与种间分化的关联是物种形成研究中的核心难题之一。相较于常染色体，X染色体被认为演化速率更快，且对生殖屏障的形成贡献更为突出；通过比较物种内及物种间X染色体与常染色体标记的遗传变异，可阐明塑造基因组变异的演化进程。本研究对覆盖异域分布与同域分布区域的16个种群样带开展了限制性位点相关DNA测序（RADseq），实验对象为两种近缘澳大利亚蟋蟀：*Teleogryllus commodus*与*T. oceanicus*。该类群是性选择研究的经典模式系统，同时也是霍尔丹法则（Haldane's rule）罕见的例外类群：其杂交雌性个体不育。尽管这两个物种在野外生境重叠区域共存，且可在实验室中杂交，但本研究未发现近期基因渐渗的相关证据。相较于常染色体位点，推定的X连锁位点在物种间展现出更高的遗传分化水平。然而，物种内的种群分化在X连锁标记中却意外低于常染色体标记，且X染色体与常染色体的相对遗传多样性较中性预期更高。两个物种种群均展现出近期种群扩张的基因组信号，但该信号不足以解释X染色体与常染色体相对遗传多样性偏高的现象。取而代之的是，X染色体上过剩的多态性可更好地通过提升X染色体相对有效种群大小（effective population size）的性别偏向过程加以解释，例如雄性间性选择强度的种间差异。综上，X染色体与常染色体位点在多样性与分化水平上呈现出的对立模式，表明性连锁基因在该系统的物种边界维持中发挥了更为关键的作用。