Department of Animal and Plant Sciences, University of Sheffield, Sheffield, UK.
Institute of Science and Technology Austria (IST Austria), Klosterneuburg, Austria.
Philos Trans R Soc Lond B Biol Sci. 2020 Aug 31;375(1806):20190545. doi: 10.1098/rstb.2019.0545. Epub 2020 Jul 13.
The evolution of strong reproductive isolation (RI) is fundamental to the origins and maintenance of biological diversity, especially in situations where geographical distributions of taxa broadly overlap. But what is the history behind strong barriers currently acting in sympatry? Using whole-genome sequencing and single nucleotide polymorphism genotyping, we inferred (i) the evolutionary relationships, (ii) the strength of RI, and (iii) the demographic history of divergence between two broadly sympatric taxa of intertidal snail. Despite being cryptic, based on external morphology, and differ in their mode of female reproduction (egg-laying versus brooding), which may generate a strong post-zygotic barrier. We show that egg-laying and brooding snails are closely related, but genetically distinct. Genotyping of 3092 snails from three locations failed to recover any recent hybrid or backcrossed individuals, confirming that RI is strong. There was, however, evidence for a very low level of asymmetrical introgression, suggesting that isolation remains incomplete. The presence of strong, asymmetrical RI was further supported by demographic analysis of these populations. Although the taxa are currently broadly sympatric, demographic modelling suggests that they initially diverged during a short period of geographical separation involving very low gene flow. Our study suggests that some geographical separation may kick-start the evolution of strong RI, facilitating subsequent coexistence of taxa in sympatry. The strength of RI needed to achieve sympatry and the subsequent effect of sympatry on RI remain open questions. This article is part of the theme issue 'Towards the completion of speciation: the evolution of reproductive isolation beyond the first barriers'.
强烈生殖隔离（RI）的进化是生物多样性起源和维持的基础，尤其是在分类单元的地理分布广泛重叠的情况下。但是，目前在同域中起作用的强大障碍背后的历史是什么？通过全基因组测序和单核苷酸多态性基因分型，我们推断了（i）进化关系，（ii）RI 的强度，以及（iii）两种广泛同域潮间带蜗牛之间的分歧的种群历史。尽管在外部形态上是隐蔽的，并且在雌性繁殖方式（产卵与孵育）上有所不同，但这可能产生强烈的合子后障碍。我们表明，产卵和孵育的蜗牛密切相关，但在遗传上是不同的。对来自三个地点的 3092 只蜗牛进行基因分型未能恢复任何近期的杂种或回交个体，证实了 RI 的强度。然而，存在非常低水平的不对称基因渗入的证据，表明隔离仍然不完全。这些种群的种群动态分析进一步支持了强烈、不对称 RI 的存在。尽管这些分类单元目前广泛同域，但种群动态模型表明，它们最初在涉及非常低基因流的短暂地理分离期间发生分歧。我们的研究表明，一些地理隔离可能会启动强烈 RI 的进化，促进分类单元在同域中的后续共存。实现同域所需的 RI 强度以及同域对 RI 的后续影响仍然是悬而未决的问题。本文是主题问题“迈向完成物种形成：生殖隔离进化超越第一道障碍”的一部分。
