Data from: Evolution in situ: hybrid origin and establishment of willows (Salix L.) on alpine glacier forefields

Little attention has been paid to the evolutionary consequences of the colonizing dynamics and succession processes following glacier retreat. Here we studied hybrid populations that have recently formed and established on glacier forefields of the European Alps owing to secondary contact of a lowland colonizer with a subalpine species. We analyzed the composition of two hybrid populations between Salix purpurea and Salix helvetica with nine microsatellite markers by using Bayesian methods (structure and NewHybrids), and simulations. We also studied niche differentiation between the hybrids and the parental species based on indicator values, soil pH and water retention potential measurements. Allelic structure of hybrids confirms the assumed parentage and in situ origin of the crosses on two independent sites within the last decades. Both hybrid populations comprised F1 and later generation hybrids (F2 and backcrosses), confirming hybrid fertility. The parental species showed significant differences in niche characteristics for temperature, soil pH, nutrients and moisture. Remarkably, the hybrids exhibited a higher tolerance to cold temperatures, nutrient-poor and acidic soils than either parent. Our results show that willow hybrids originated after glacier retreat and have established persistent populations within a few decades. One factor contributing to hybrid establishment in sympatry with their parents is their ability to occupy more extreme niches than either parental species within a mosaic-like pattern of microhabitats on the forefield. Introgression and/or transgressive segregation may have resulted in novel genotypes that are able to expand the ecological spectrum of either parent.冰川退缩后的定殖动态与演替过程所引发的演化效应，迄今尚未得到足够关注。本研究聚焦欧洲阿尔卑斯山冰川前缘近期形成并定居的杂交种群——该类种群源于一个低地定殖物种与一个亚高山物种的二次接触。研究团队利用9个微卫星标记（microsatellite markers），通过贝叶斯方法（Bayesian methods，包含Structure与NewHybrids两款软件）及模拟分析，解析了紫柳（Salix purpurea）与瑞士柳（Salix helvetica）间两个杂交种群的遗传组成；同时基于生态位指示值、土壤pH值及持水潜能（water retention potential）测定数据，探究了杂交种群与亲本物种间的生态位分化（niche differentiation）。杂交种群的等位基因结构（allelic structure）证实了其假定的亲本来源与原位起源（in situ origin），表明该杂交事件在近数十年间于两个独立样地完成。两个杂交种群均包含子一代（F1）以及子二代（F2）、回交后代（backcrosses）等后续世代杂交个体，证实了杂交可育性（hybrid fertility）。亲本物种在温度、土壤pH值、养分及湿度相关的生态位特征上存在显著差异。值得注意的是，相较于任一亲本，杂交种群对低温、贫瘠土壤与酸性土壤展现出更高的耐受性。本研究结果显示，柳属杂交种群起源于冰川退缩之后，并在数十年内形成了稳定存续的种群。杂交种群能够与亲本物种同域共存（sympatry）并成功定居，其中一个关键驱动因素为：相较于亲本物种，杂交种群可占据冰川前缘微生境镶嵌格局中更为极端的生态位。基因渐渗（introgression）和/或超亲分离（transgressive segregation）可能催生了全新的基因型，使得杂交种群能够拓展任一亲本物种的生态幅（ecological spectrum）。