Data from: Rapid evolution of an adaptive cyanogenesis cline in introduced North American white clover (Trifolium repens L.)

White clover is polymorphic for cyanogenesis (HCN production after tissue damage), and this herbivore defense polymorphism has served as a classic model for studying adaptive variation. The cyanogenic phenotype requires two interacting biochemical components; the presence/absence of each component is controlled by a simple Mendelian gene (Ac/ac and Li/li). Climate-associated cyanogenesis clines occur in both native (Eurasian) and introduced populations worldwide, with cyanogenic plants predominating in warmer locations. Moreover, previous studies have suggested that epistatic selection may act within populations to maintain cyanogenic (AcLi) plants and acyanogenic plants that lack both components (acli plants) at the expense of plants possessing a single component (Acli and acLi plants). Here we examine the roles of selection, gene flow, and demography in the evolution of a latitudinal cyanogenesis cline in introduced North American populations. Using 1,200 plants sampled across a 1,650 km transect, we determine the distribution of cyanogenesis variation across the central U.S. and investigate whether clinal variation is adaptive or an artifact of population introduction history. We also test for evidence of epistatic selection. We detect a clear latitudinal cline, with cyanogenesis frequencies increasing from 11% to 86% across the transect. Population structure analysis using nine microsatellite loci indicates that the cline is adaptive and not a byproduct of demographic history. However, we find no evidence for epistatic selection within populations. Our results provide strong evidence for rapid adaptive evolution in these introduced populations, and they further suggest that the mechanisms maintaining adaptive variation may vary among populations of a species.

白车轴草（White clover）存在产氰作用（cyanogenesis，即组织损伤后释放氰化氢）的多态性，这种草食动物防御多态性已成为研究适应性变异的经典模型。产氰表型需要两种相互作用的生化组分；每种组分的有无分别由一对孟德尔基因（Mendelian gene）调控，即Ac/ac与Li/li。与气候相关的产氰渐变群（cline）在全球的原生（欧亚大陆）种群与引入种群中均有分布，产氰型植物在温暖生境中占主导地位。此外，既往研究提示，种群内可能存在上位性选择（epistatic selection）：通过牺牲仅携带单一组分的植物（Acli与acLi型），维持同时携带两种组分的产氰型（AcLi）以及完全缺失两种组分的非产氰型（acli）植物。本研究旨在探究选择、基因流与种群历史动态在北美引入种群的纬度产氰渐变群演化中的作用。我们对横跨1650公里样带的1200株植物进行采样，明确美国中部地区产氰变异的分布格局，并探究该渐变群变异是适应性演化的结果，还是种群引入历史带来的人为假象。本研究同时检验了种群内上位性选择的存在证据。结果显示，样带内存在显著的纬度渐变群：产氰型植物频率从11%升至86%。基于9个微卫星位点（microsatellite loci）的种群结构分析表明，该渐变群为适应性演化的产物，而非种群历史动态的副产物。但我们未发现种群内存在上位性选择的证据。本研究结果为这些引入种群的快速适应性演化提供了有力证据，同时进一步表明，维持物种内适应性变异的机制可能在不同种群间存在差异。