Cities are emerging as models for addressing the fundamental question of whether populations evolve in parallel to similar environments. Here, we examine the environmental factors that drive the evolution of parallel urban‐rural clines in a Mendelian trait—the cyanogenic antiherbivore defense of white clover (Trifolium repens). Previous work suggested urban‐rural gradients in frost and snow depth could drive the evolution of reduced hydrogen cyanide (HCN) frequencies in urban populations. Here, we sampled over 700 urban and rural clover populations across 16 cities along a latitudinal transect in eastern North America. In each population, we quantified changes in the frequency of genotypes that produce HCN, and in a subset of the cities we estimated the frequency of the alleles at the two genes (CYP79D15 and Li) that epistatically interact to produce HCN. We then tested the hypothesis that cold climatic conditions are necessary for the evolution of cyanogenesis clines by comparing the strength of clines among cities located along a latitudinal gradient of winter temperature and frost exposure. Overall, half of the cities exhibited urban‐rural clines in the frequency of HCN, whereby urban populations evolved lower HCN frequencies. Clines did not evolve in cities with the lowest temperatures and greatest snowfall, supporting the hypothesis that snow buffers plants against winter frost and constrains the formation of clines. By contrast, the strongest clines occurred in the warmest cities where snow and frost are rare, suggesting that alternative selective agents are maintaining clines in warmer cities. Some clines were driven by evolution at only CYP79D15, consistent with stronger and more consistent selection on this locus than on Li. Together, our results demonstrate that urban environments often select for similar phenotypes, but different selective agents and targets underlie the evolutionary response in different cities.

中文翻译：

---

沿着纬度梯度预测孟德尔多态性中的城乡界线强度

城市正在成为解决人口是否与类似环境平行发展这一基本问题的模型。在这里，我们研究了环境因素，这些因素驱动孟德尔特征的平行城乡血统演变-白三叶草（Trifolium repens）的生氰抗草食动物防御能力。先前的工作表明，霜冻和积雪深度的城乡梯度可能会推动城市人口氰化氢（HCN）减少频率的演变。在这里，我们在北美东部沿纬度样带的16个城市中，对700多个城乡三叶草种群进行了采样。在每个人口中，我们量化了产生HCN的基因型频率的变化，并且在部分城市中，我们估算了这两个基因的等位基因频率（CYP79D15和Li）上位相互作用产生HCN。然后，我们通过比较沿冬季温度和霜冻暴露纬度梯度分布的城市之间的气候变化强度，验证了寒冷气候条件对于蓝藻变化的演化所必需的假设。总体而言，一半城市的HCN频率呈城乡变化趋势，因此城市人口的HCN频率较低。在气温最低，降雪量最大的城市中，克莱恩氏菌并未进化，这支持了这样的假说：积雪可以缓冲植物免受冬季霜冻的影响，并限制了氏菌的形成。相比之下，最强的上升趋势发生在最罕见的雪和霜冻最温暖的城市，这表明替代选择剂在较温暖的城市保持着上升趋势。有些血统只是受到进化的驱使CYP79D15，与在Li上相比，在该基因座上的选择更加强大和一致。总之，我们的结果表明，城市环境通常会选择相似的表型，但是不同城市的进化反应是不同选择因子和目标的基础。