Data from: Searching for the bull's-eye: agents and targets of selection vary among geographically disparate cyanogenesis clines in white clover (Trifolium repens L.)

The recurrent evolution of adaptive clines within a species can be used to elucidate the selective factors and genetic responses that underlie adaptation. White clover is polymorphic for cyanogenesis (HCN release with tissue damage), and climate-associated cyanogenesis clines have evolved throughout the native and introduced species range. This polymorphism arises through two independently segregating Mendelian polymorphisms for the presence/absence of two required components: cyanogenic glucosides and their hydrolyzing enzyme linamarase. Cyanogenesis is commonly thought to function in herbivore defense; however, the individual cyanogenic components may also serve other physiological functions. To test whether cyanogenesis clines have evolved in response to the same selective pressures acting on the same genetic targets, we examined cyanogenesis cline shape and its environmental correlates in three world regions: southern New Zealand, the central U.S., and the U.S. Pacific Northwest. For some regional comparisons, cline shapes are remarkably similar despite large differences in the spatial scales over which clines occur (40- 1,600 km). However, we also find evidence for major differences in both the agents and targets of selection among the sampled clines. Variation in cyanogenesis frequency is best predicted using a combination of minimum winter temperature and aridity variables. Together our results provide evidence that recurrent adaptive clines do not necessarily reflect shared adaptive processes.一个物种种内适应性渐变群（adaptive clines）的重复演化，可用于阐明适应背后的选择因子与遗传响应机制。白三叶（White clover）因产氰作用（cyanogenesis，即组织损伤时释放氰化氢HCN）呈现多态性，且在其自然分布区与引种分布区内均演化出与气候相关的产氰作用渐变群。该多态性源于两个独立分离的孟德尔多态性，对应两种必需组分的有无：氰化葡萄糖苷（cyanogenic glucosides）及其水解酶亚麻苦苷酶（linamarase）。产氰作用通常被认为在植食动物防御中发挥功能，但单个产氰组分可能还承担其他生理功能。为检验产氰作用渐变群是否响应了作用于相同遗传靶点的相同选择压力，我们在全球三个区域——新西兰南部、美国中部以及美国太平洋西北地区——考察了产氰作用渐变群的形态特征及其环境关联因子。部分区域对比显示，尽管渐变群发生的空间尺度差异悬殊（40~1600千米），但渐变群形态却极为相似。然而，我们也发现，在所采样的渐变群之间，选择作用的因子与靶点均存在显著差异的证据。产氰作用频率的变异，最优可通过冬季最低温与干旱度变量的组合进行预测。综合来看，我们的研究结果表明，重复出现的适应性渐变群未必对应共通的适应性演化过程。