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）多态性——即组织损伤后可产生氰化氢（HCN），这种针对草食动物的防御多态性已成为适应性变异研究的经典模型。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简单的孟德尔基因（Mendelian gene）调控，即Ac/ac与Li/li。全球范围内的原生（欧亚大陆）与引入种群中均存在与气候相关的产氰梯度，温暖生境中以产氰型植株占优势。此外，既往研究提示，种群内可能存在上位选择（epistatic selection），通过牺牲仅携带单一功能组分的植株（Acli与acLi型），维持产氰型（AcLi型）与缺乏两种组分的无氰型（acli型）植株共存。本研究聚焦北美引入种群的纬度梯度产氰变异演化，探讨选择作用、基因流与种群历史动态在其中的具体角色。本研究通过横跨1650公里的样带采集的1200株植株，明确了美国中部地区的产氰变异分布格局，并探究该梯度变异究竟是适应性演化的结果，还是种群引入历史带来的非适应性假象。同时，本研究还检验了种群内上位选择存在的证据。研究结果显示存在显著的纬度梯度：样带内产氰植株频率从11%攀升至86%。基于9个微卫星位点（microsatellite loci）的种群结构分析表明，该产氰梯度属于适应性演化，而非种群历史动态的副产物。但本研究未检测到种群内存在上位选择的证据。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支撑，同时也表明，维持物种内适应性变异的演化机制可能在不同种群间存在差异。