Department of Plant Sciences, University of Oxford, South Parks Road, Oxford, OX1 3RB, United Kingdom; Centre for Organismal Studies, Department of Biodiversity and Plant Systematics, University of Heidelberg, D-69120, Heidelberg, Germany.
Evolution. 2013 Oct;67(10):3032-42. doi: 10.1111/evo.12157. Epub 2013 Jun 21.
The hybrid zone on Mount Etna (Sicily) between Senecio aethnensis and Senecio chrysanthemifolius (two morphologically and physiologically distinct species) is a classic example of an altitudinal cline. Hybridization at intermediate altitudes and gradients in phenotypic and life-history traits occur along altitudinal transects of the volcano. The cline is considered to be a good example of ecological selection with species differences arising by divergent selection opposing gene flow. However, the possibility that the cline formed from recent secondary contact following an allopatric phase is difficult to exclude. We demonstrate a recent split between S. aethnensis and S. chrysanthemifolius (as recent as ∼32,000 years ago) and sufficient gene flow (2Nm > 1) to have prevented divergence (implicating a role for diversifying selection in the maintenance of the cline). Differentially expressed genes between S. aethnensis and S. chrysanthemifolius exhibit significantly higher genetic divergence relative to "expression invariant" controls, suggesting that species differences may in part be mediated by divergent selection on differentially expressed genes involved with altitude-related adaptation. The recent split time and the absence of fixed differences between these two ecologically distinct species suggest the rapid evolution to an altitudinal cline involving selection on both sequence and expression variation.
埃特纳火山（西西里岛）上的 Senecio aethnensis 和 Senecio chrysanthemifolius 之间的杂交区（两个在形态和生理上明显不同的物种）是一个经典的海拔渐变的例子。在火山的海拔横剖面上，中海拔的杂交和表型及生活史特征的梯度变化都发生了。该渐变被认为是生态选择的一个很好的例子，物种差异是由相反的基因流的分歧选择产生的。然而，很难排除渐变是由异域阶段后的近期二次接触形成的可能性。我们证明了 S. aethnensis 和 S. chrysanthemifolius 之间的最近分裂（最近的时间大约是 32000 年前）和足够的基因流（2Nm > 1），以防止分歧（暗示了多样化选择在渐变的维持中的作用）。S. aethnensis 和 S. chrysanthemifolius 之间差异表达的基因与“表达不变”的对照相比，表现出显著更高的遗传分化，这表明物种差异可能部分是由与海拔相关的适应相关的差异表达基因的分歧选择介导的。最近的分裂时间和这两个生态明显不同的物种之间没有固定差异表明，快速进化到涉及序列和表达变异选择的海拔渐变。
