Data from: Recent ecological selection on regulatory divergence is shaping clinal variation in Senecio on Mount Etna

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.

西西里岛埃特纳火山上的千里光（Senecio aethnensis）与菊叶千里光（Senecio chrysanthemifolius）之间的杂交带（hybrid zone）是海拔梯度渐变群（altitudinal cline）的经典研究案例，二者为形态与生理特征均存在显著差异的两个物种。在该火山的海拔样带（altitudinal transects）上，中间海拔区域存在自然杂交现象，同时表型（phenotypic）与生活史性状（life-history traits）的梯度变化沿样带分布。该渐变群被视为生态选择（ecological selection）的典型范例，物种间的差异由趋异选择（divergent selection）对抗基因流（gene flow）所塑造。然而，该渐变群也可能形成于异域分化阶段（allopatric phase）后的近期次生接触（secondary contact），这一假说难以被完全排除。本研究证实，S. aethnensis与S. chrysanthemifolius的分化时间极近（仅约3.2万年前），且存在足够强的基因流（2Nm > 1），本应阻碍种群趋异，这表明趋异选择在维持该渐变群的过程中发挥了关键作用。相较于“表达恒定（expression invariant）”的对照基因，两个物种间的差异表达基因展现出显著更高的遗传分化（genetic divergence），这提示物种间的差异可能部分由参与海拔适应的差异表达基因受到趋异选择所介导。二者的近期分化时间以及未出现固定遗传差异的现象，表明这两个生态特征迥异的物种，通过针对序列变异（sequence variation）与表达变异（expression variation）的定向选择，快速演化形成了该海拔梯度渐变群。