Data from: Hybridization during altitudinal range shifts: nuclear introgression leads to extensive cyto-nuclear discordance in the fire salamander

Ecological models predict that, in the face of climate change, taxa occupying steep altitudinal gradients will shift their distributions, leading to the contraction or extinction of the high-elevation (cold-adapted) taxa. However, hybridization between eco-morphologically divergent taxa commonly occurs in nature and may lead to alternative evolutionary outcomes, such as genetic merger or gene flow at specific genes. We evaluate this hypothesis by studying patterns of divergence and gene flow across three replicate contact zones between high- and low-elevation ecomorphs of the fire salamander (Salamandra salamandra) that have experienced altitudinal range shifts over the current postglacial period. Strong population structure with high genetic divergence in mitochondrial DNA suggests that vicariant evolution has occurred over several glacial-interglacial cycles, and that it has led to cryptic differentiation within ecomorphs. In current parapatric boundaries we do not find evidence for local extinction and replacement upon postglacial expansion. Instead parapatric taxa recurrently show discordance between mitochondrial and nuclear markers, suggesting nuclear mediated gene flow across contact zones. Isolation with migration models support this hypothesis by showing significant gene flow across all five parapatric boundaries. Together, our results suggest that, while some genomic regions, such as the mitochondria, may follow morphologic species traits and retreat to isolated mountain tops, other genomic regions, such as nuclear markers, may flow across parapatric boundaries, sometimes leading to a complete genetic merger. We show that despite high ecologic and morphologic divergence over prolonged periods of time, hybridization allows for evolutionary outcomes alternative to extinction and replacement of taxa in response to climate change.

生态模型预测，在气候变化背景下，占据陡峭海拔梯度的生物类群将发生分布范围转移，导致高海拔（耐寒）类群的分布收缩乃至灭绝。然而，自然界中普遍存在生态形态分化类群间的杂交现象，这可能催生另类演化结局，例如特定基因的遗传融合或基因流事件。我们以火蝾螈（Salamandra salamandra）的高低海拔生态型为研究对象，通过分析其三个重复接触带内的分化模式与基因流情况，对该假说开展验证——这些类群在当前冰后期时期已出现海拔范围的迁移。线粒体DNA（mitochondrial DNA）中呈现出高遗传分化的显著种群结构，这表明多次冰期-间冰期循环驱动了异域歧异演化，并使得生态型内部产生了隐秘分化。在当前的邻域边界处，我们未发现冰后期扩张过程中存在局部灭绝与物种替代的证据。与之相反，邻域类群反复在线粒体与核标记之间展现出遗传不一致性，这暗示存在跨越接触带的核介导基因流。隔离与迁移模型（isolation with migration models）的结果支持这一假说，其显示所有五个邻域边界均存在显著的基因流。综合来看，我们的研究结果表明：尽管线粒体这类基因组区域可能遵循形态物种性状，退缩至孤立的山顶区域，但核标记这类其他基因组区域却可跨越邻域边界发生基因流，有时甚至会引发完全的遗传融合。本研究证实，即便类群间历经长期的高度生态与形态分化，杂交仍可在气候变化的背景下，催生不同于类群灭绝与替代的演化结局。