Data from: Hybrid asexuality as a primary postzygotic barrier between nascent species: on the interconnection between asexuality, hybridization and speciation

Although sexual reproduction is ubiquitous throughout nature, the molecular machinery behind it has been repeatedly disrupted during evolution, leading to the emergence of asexual lineages in all eukaryotic phyla. Despite intensive research, little is known about what causes the switch from sexual reproduction to asexuality. Interspecific hybridization is one of the candidate explanations but the reasons for the apparent association between hybridization and asexuality remain unclear. In this study we combined cross-breeding experiments with population genetic and phylogenomic approaches to reveal the history of speciation and asexuality evolution in European spined loaches (Cobitis). Contemporary species readily hybridize in hybrid zones, but produce infertile males and fertile but clonally reproducing females that cannot mediate introgressions. However, our analysis of exome data indicates that intensive gene flow between species has occurred in the past. Crossings among species with various genetic distances showed that, while distantly related species produced asexual females and sterile males, closely related species produce sexually reproducing hybrids of both sexes. Our results suggest that hybridization leads to sexual hybrids at the initial stages of speciation, but as the species diverge further, the gradual accumulation of reproductive incompatibilities between species could distort their gametogenesis towards asexuality. Interestingly, comparative analysis of published data revealed that hybrid asexuality generally evolves at lower genetic divergences than hybrid sterility or inviability. Given that hybrid asexuality effectively restricts gene flow, it may establish a primary reproductive barrier earlier during diversification than other ‘classical’ forms of postzygotic incompatibilities. Hybrid asexuality may thus indirectly contribute to the speciation process.尽管有性生殖（sexual reproduction）在自然界中普遍存在，但其背后的分子调控机制在演化历程中反复遭到破坏，进而在所有真核生物门（eukaryotic phyla）中催生了无性生殖演化支。尽管已有大量深入研究，学界对驱动有性生殖向无性生殖转变的诱因仍知之甚少。种间杂交（interspecific hybridization）是候选解释之一，但杂交与无性生殖之间为何存在显著关联，其背后的机制仍未阐明。本研究将杂交育种实验与种群遗传学、系统发育基因组学（phylogenomic）方法相结合，旨在阐明欧洲棘鳅（*Cobitis*）的物种形成与无性生殖演化历程。现生物种在杂交带中可顺利发生杂交，但会产生不育雄性与可育但以克隆方式繁殖的雌性，此类雌性无法介导基因渐渗（introgression）。然而，我们对外显子组（exome）数据的分析显示，物种间在历史上曾发生过频繁的基因交流。对不同遗传距离物种间的杂交实验结果显示，亲缘关系较远的物种会产生无性生殖雌性与不育雄性，而亲缘关系较近的物种则会产生可进行有性生殖的两性杂交后代。本研究结果表明，在物种形成的初始阶段，杂交会产生有性生殖的杂交后代；但随着物种间分化程度进一步加深，物种间生殖不相容性的逐步积累可能会使其配子发生（gametogenesis）向无性生殖方向偏离。值得注意的是，对已发表数据的比较分析显示，相较于杂交不育或杂交致死，杂交无性生殖通常在更低的遗传分化水平下演化形成。鉴于杂交无性生殖可有效限制基因交流，其在物种分化过程中可能比其他“经典”合子后生殖不相容性（postzygotic incompatibilities）更早形成主要的生殖隔离屏障。因此，杂交无性生殖可能会间接推动物种形成进程。