Recently diverged species present particularly informative systems for studying speciation and maintenance of genetic divergence in the face of gene flow. We investigated speciation in two closely related Senecio species, S. aethnensis and S. chrysanthemifolius, which grow at high and low elevations, respectively, on Mount Etna, Sicily and form a hybrid zone at intermediate elevations. We used a newly generated genome-wide single nucleotide polymorphism (SNP) dataset from 192 individuals collected over 18 localities along an elevational gradient to reconstruct the likely history of speciation, identify highly differentiated SNPs, and estimate the strength of divergent selection. We found that speciation in this system involved heterogeneous and bidirectional gene flow along the genome, and species experienced marked population size changes in the past. Furthermore, we identified highly-differentiated SNPs between the species, some of which are located in genes potentially involved in ecological differences between species (such as photosynthesis and UV response). We analysed the shape of these SNPs' allele frequency clines along the elevational gradient. These clines show significantly variable coincidence and concordance, indicative of the presence of multifarious selective forces. Selection against hybrids is estimated to be very strong (0.16-0.78) and one of the highest reported in literature. The combination of strong cumulative selection across the genome and previously identified intrinsic incompatibilities probably work together to maintain the genetic and phenotypic differentiation between these species - pointing to the importance of considering both intrinsic and extrinsic factors when studying divergence and speciation.

中文翻译：

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在两个密切相关的豚草物种的多个基因座上的强差异选择，这些物种适合于埃特纳火山的高海拔和低海拔。

最近分化的物种提供了特别有用的系统，用于研究面对基因流的物种形成和遗传多样性的维持。我们调查了两个紧密相关的千里光物种的物种形成情况，即S. aethnensis和S. chsssanthemifolius，它们分别在高海拔和低海拔上生长在西西里岛的埃特纳火山上，并在中等海拔上形成杂种带。我们使用了一个新的全基因组范围内的单核苷酸多态性（SNP）数据集，该数据集来自沿海拔梯度分布在18个位置的192个个体，以重建可能的物种形成历史，识别高度分化的SNP，并估计差异选择的强度。我们发现该系统中的物种形成涉及沿基因组的异质和双向基因流动，和物种在过去经历了明显的种群大小变化。此外，我们确定了物种之间的高度分化SNP，其中一些位于可能与物种之间的生态差异（例如光合作用和紫外线响应）有关的基因中。我们分析了这些SNPs等位基因频率沿海拔梯度的形状。这些谱线显示出明显不同的巧合和一致性，表明存在多种选择力。估计对杂种的选择非常强（0.16-0.78），是文献中报道的最高的之一。