Data from: Distinguishing contemporary hybridization from past introgression with postgenomic ancestry-informative SNPs in strongly differentiated Ciona species

Biological introductions bring into contact species that can still hybridize. The evolutionary outcomes of such secondary contacts may be diverse (e.g. adaptive introgression from or into the introduced species) but are not yet well examined in the wild. The recent secondary contact between the non-native sea squirt Ciona robusta (formerly known as C. intestinalis type A) and its native congener C. intestinalis (formerly known as C. intestinalis type B), in the Western English Channel, provides an excellent case study to examine. To examine contemporary hybridization between the two species, we developed a panel of 310 ancestry-informative SNPs from a population transcriptomic study. Hybridization rates were examined on 449 individuals sampled in eight sites from the sympatric range and five sites from allopatric ranges. The results clearly showed an almost complete absence of contemporary hybridization between the two species in syntopic localities, with only one-first-generation hybrid and no other genotype compatible with recent backcrosses. Despite the almost lack of contemporary hybridization, shared polymorphisms were observed in sympatric and allopatric populations of both species. Furthermore, one allopatric population from SE Pacific exhibited a higher rate of shared polymorphisms compared to all other C. robusta populations. Altogether, these results indicate that the observed level of shared polymorphism is more probably the outcome of ancient gene flow spread afterwards at a worldwide scale. They also emphasize efficient reproductive barriers preventing hybridization between introduced and native species, which suggests hybridization should not impede too much the expansion and the establishment of the non-native species in its introduction range.

生物引种会使仍可发生杂交的物种产生接触。此类二次接触所带来的进化结果可能多种多样（例如，发生于引种物种与本土物种之间或物种内部的适应性基因渐渗），但在自然环境中尚未得到充分研究。西英吉利海峡区域内，非本土海鞘Ciona robusta（原称C. intestinalis A型）与其本土近缘物种Ciona intestinalis（原称C. intestinalis B型）之间新近发生的二次接触，为该类研究提供了极佳的案例体系。为探究这两个物种间的当代杂交事件，本研究基于群体转录组学分析，开发了一套包含310个祖先信息单核苷酸多态性（ancestry-informative SNPs）的标记面板。研究团队对采自同域分布区8个采样点以及异域分布区5个采样点的449份个体进行了杂交率检测。结果清晰表明，在同域栖息生境中，这两个物种几乎完全不存在当代杂交现象，仅检测到1例第一代杂交个体，未发现与近期回交相符的其他基因型。尽管当代杂交事件几乎不存在，但在两个物种的同域与异域种群中均检测到了共享多态性。此外，与其他所有Ciona robusta种群相比，采自东南太平洋的一个异域种群展现出更高的共享多态性比例。综合来看，上述结果表明，所观测到的共享多态性水平更可能是远古基因流后续在全球范围内扩散所导致的结果。本研究同时证实，引种物种与本土物种之间存在有效的生殖隔离屏障，这意味着杂交不会显著阻碍非本土物种在其引种区域内的扩散与定殖。