CIBIO, Centro de Investigação em Biodiversidade e Recursos Genéticos, InBIO, Laboratório Associado, Universidade do Porto, Vairão, Portugal.
Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2TN, UK.
BMC Evol Biol. 2020 Feb 10;20(1):23. doi: 10.1186/s12862-019-1561-6.
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.
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.
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.
扁玉螺（Littorina fabalis）和光滑玉螺（L. obtusata）是两种分布广泛的姐妹种，广泛分布于北大西洋沿岸，具有很高的潜力可以帮助我们了解潮间带生态物种形成的过程。然而，它们在分化过程中是否发生了基因流仍然存在争议。对这些物种遗传多样性的全面评估也缺乏，它们的主要冰川避难所和扩散障碍仍然在很大程度上未知。为了填补这些空白，我们对这两种扁玉螺进行了测序，分析了它们在分布范围内的系统地理学。
我们鉴定了两个主要的分支，这些分支主要由与 L. obtusata 和 L. fabalis 相对应的种特异性单倍型组成，分别得到中度到强烈的支持。重要的是，在伊比利亚和北欧-中欧，都更好地支持了两种物种之间存在基因流的分化模型。在 L. fabalis 中检测到三个线粒体分支，在 L. obtusata 中检测到两个分支，伊比利亚和其余种群之间存在强烈的分化。每个物种内遗传方差的最大组成部分是由与这些分支相关的地理区域之间的差异解释的。我们的数据表明，两种扁玉螺的种内遗传多样性总体相似，伊比利亚种群的多样性往往低于北欧-中欧种群。
对这对姐妹种的系统地理学分析支持分化伴随着基因流。因此，这个系统为我们提供了研究基因流和自然选择在多样化过程中的贡献的机会。不同分支的分布表明，这两个物种在伊比利亚和北欧-中欧都存在冰川避难所，在这些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并不完全具有结论性，但在伊比利亚观察到的较低多样性可能反映了它们在当前间冰期分布范围的南部边缘的边缘条件。
