Department of Biology, Washington University in St Louis, One Brookings Drive, Campus Box 1137, St. Louis, MO 63130, USA.
Mol Ecol. 2012 May;21(10):2455-68. doi: 10.1111/j.1365-294X.2012.05486.x. Epub 2012 Feb 16.
White clover is polymorphic for cyanogenesis (HCN production after tissue damage), and this herbivore defence 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 1145 plants sampled across a 1650 km transect, we determine the distribution of cyanogenesis variation across the central United States and investigate whether clinal variation is adaptive or an artefact of population introduction history. We also test for the 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 by-product 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.
白车轴草存在氰苷生物合成（组织损伤后 HCN 的产生）的多态性，这种食草动物防御多态性已成为研究适应性变异的经典模式。产生氰苷的表型需要两个相互作用的生化成分；每个成分的存在/缺失由一个简单的孟德尔基因（Ac/ac 和 Li/li）控制。与气候相关的氰苷表型梯度在原产（欧亚）和引入的世界各地的种群中都存在，在温暖的地方，氰苷植物占主导地位。此外，以前的研究表明，上位性选择可能在种群内发挥作用，以维持以氰苷（AcLi）植物和缺乏两种成分的非氰苷植物（acli 植物）为代价，而牺牲了只拥有一种成分的植物（Acli 和 acLi 植物）。在这里，我们研究了选择、基因流和人口统计学在引入的北美种群中纬度氰苷梯度进化中的作用。使用在 1650 公里的样带上采集的 1145 株植物，我们确定了美国中部氰苷变异的分布，并研究了梯度变异是否是适应性的，还是种群引入历史的人为产物。我们还检验了上位性选择的证据。我们检测到一个明显的纬度梯度，在样带上，氰苷的频率从 11%增加到 86%。使用九个微卫星位点的种群结构分析表明，梯度是适应性的，而不是人口历史的副产品。然而，我们没有在种群内发现上位性选择的证据。我们的研究结果为这些引入种群的快速适应性进化提供了有力的证据，并且进一步表明，维持适应性变异的机制可能在物种的不同种群之间存在差异。
