BACKGROUND The flat periwinkles, Littorina fabalis and L. obtusata, are two sister species widely distributed throughout the Northern Atlantic shores with high potential to inform us about the process of ecological speciation in the intertidal. However, whether gene flow has occurred during their divergence is still a matter of debate. A comprehensive assessment of the genetic diversity of these species is also lacking and their main glacial refugia and dispersal barriers remain largely unknown. In order to fill these gaps, we sequenced two mitochondrial genes and two nuclear fragments to perform a phylogeographic analysis of flat periwinkles across their distribution range. RESULTS We identified two main clades largely composed by species-specific haplotypes corresponding to L. obtusata and L. fabalis, with moderate to strong support, respectively. Importantly, a model of divergence with gene flow between the two species (from L. obtusata to L. fabalis) was better supported, both in Iberia and in northern-central Europe. Three mitochondrial clades were detected within L. fabalis and two within L. obtusata, with strong divergence between Iberia and the remaining populations. The largest component of the genetic variance within each species was explained by differences between geographic regions associated with these clades. Our data suggests that overall intraspecific genetic diversity is similar between the two flat periwinkle species and that populations from Iberia tend to be less diverse than populations from northern-central Europe. CONCLUSIONS The phylogeographic analysis of this sister-species pair supports divergence with gene flow. This system thus provides us with the opportunity to study the contribution of gene flow and natural selection during diversification. The distribution of the different clades suggests the existence of glacial refugia in Iberia and northern-central Europe for both species, with a main phylogeographic break between these regions. Although the genetic diversity results are not fully conclusive, the lower diversity observed in Iberia could reflect marginal conditions at the southern limit of their distribution range during the current interglacial period.

中文翻译：

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扁平长春花，小花小立陶宛和L. obtusata的植物史。

背景技术扁平的长春花，小花小Littorina fabalis和L. obtusata是在北大西洋沿岸广泛分布的两个姊妹物种，具有很高的潜力，可以使我们了解潮间带的生态物种形成过程。然而，基因流在它们的分离过程中是否已经发生还存在争议。还缺乏对这些物种遗传多样性的全面评估，其主要的冰川避难所和传播障碍仍然未知。为了填补这些空白，我们对两个线粒体基因和两个核片段进行了测序，以对其分布范围内的扁平长春花进行系统地理分析。结果我们确定了两个主要进化枝，主要由特定物种的单倍型组成，分别对应于小扁豆和小扁豆，具有中等到强力的支持，分别。重要的是，在伊比利亚（Iberia）和欧洲中北部，这两个物种之间的基因流差异模型（从钝角梭菌到Fab。Fabalis）得到了更好的支持。在浅水L中发现了三个线粒体进化支，在扁ob中被发现了两个线粒体进化，伊比利亚菌与其余种群之间存在很大的差异。每个物种内遗传变异的最大组成部分是由与这些进化枝相关的地理区域之间的差异解释的。我们的数据表明，两个扁平长春花物种之间的总体种内遗传多样性相似，并且来自伊比利亚的种群的多样性往往不如来自中欧北部的种群。结论对这对姐妹物种的系统地理学分析支持基因流的发散。因此，该系统为我们提供了研究多样化过程中基因流和自然选择的贡献的机会。不同进化枝的分布表明，这两个物种在伊比利亚和中欧北部均存在冰川避难所，这两个地区之间存在主要的系统地理学断裂。尽管遗传多样性的结果还不能完全得出结论，但在伊比利亚地区观察到的较低多样性可能反映了当前冰期间处于其分布范围南端的边缘条件。