Data from: Higher genetic diversity in recolonized areas than in refugia of Alnus glutinosa triggered by continent-wide lineage admixture

Genetic admixture is supposed to be an important trigger of species expansions because it can create the potential for selection of genotypes suitable for new climatic conditions. Up until now, however, no continent-wide population genetic study has performed a detailed reconstruction of admixture events during natural species expansions. To fill this gap, we analysed the postglacial history of Alnus glutinosa, a keystone species of European swamp habitats, across its entire distribution range using two molecular markers, cpDNA and nuclear microsatellites. CpDNA revealed multiple southern refugia located in the Iberian, Apennine, Balkan and Anatolian Peninsulas, Corsica and North Africa. Analysis of microsatellites variation revealed three main directions of postglacial expansion: 1) from the northern part of the Iberian Peninsula to Western and Central Europe and subsequently to the British Isles, 2) from the Apennine Peninsula to the Alps, and 3) from the eastern part of the Balkan Peninsula to the Carpathians followed by expansion towards the Northern European plains. This challenges the classical paradigm that most European populations originated from refugial areas in the Carpathians. It has been shown that colonizing lineages have met several times and formed secondary contact zones with unexpectedly high population genetic diversity in Central Europe and Scandinavia. On the contrary, limited genetic admixture in southern refugial areas of A. glutinosa renders rear-edge populations in the Mediterranean region more vulnerable to extinction due to climate change.

遗传渐渗（genetic admixture）被视为物种扩张的关键驱动因子，其可为适应新气候条件的基因型筛选提供潜在可能。然而迄今为止，尚无针对自然物种扩张过程中渐渗事件的全大洲尺度种群遗传学详细重建研究。为填补这一研究空白，我们依托叶绿体DNA（cpDNA）与核微卫星两种分子标记，对欧洲沼泽生境关键物种欧洲黑桤木（*Alnus glutinosa*）在整个分布范围内的冰后历史展开了分析。cpDNA分析揭示，伊比利亚半岛、亚平宁半岛、巴尔干半岛、安纳托利亚半岛、科西嘉岛及北非区域存在多处南部冰期避难所（refugia）。核微卫星变异分析则揭示了三条主要冰后扩张路径：1） 从伊比利亚半岛北部向西欧、中欧扩张，随后拓展至不列颠群岛；2） 从亚平宁半岛向阿尔卑斯山脉扩张；3） 从巴尔干半岛东部向喀尔巴阡山脉扩张，继而向北欧平原拓展。该结果挑战了"多数欧洲种群起源于喀尔巴阡山脉避难所"的经典范式。研究证实，拓殖谱系多次相遇，在中欧与斯堪的纳维亚地区形成了种群遗传多样性异常丰富的次生接触带。与之相对，欧洲黑桤木南部避难所区域的遗传渐渗程度有限，致使地中海地区的后缘种群更易因气候变化走向灭绝。