Urban hubs of connectivity: contrasting patterns of gene flow within and among cities in the Western black widow spider

As urbanization drastically alters the natural landscape and generates novel habitats within cities, the potential for changes to gene flow for urban-dwelling species increases. The Western black widow spider (Latrodectus hesperus) is a medically-relevant urban adapter pest species, for which we have previously identified population genetic signatures consistent with urbanization facilitating gene flow, likely due to human-mediated transport. Here, in an analysis of 1.9 million genomewide SNPs, we contrast broad-scale geographic analyses of 10 urban and 11 non-urban locales with fine-scale within-city analyses including 30 urban locales across the Western U.S. These hierarchical datasets enable us to test hypotheses of how urbanization impacts multiple urban cities and their genetic connectivity at different spatial scales. Coupled fine-scale and broad-scale analyses reveal contrasting patterns of high and low genetic differentiation among locales within cities as a result of low and high genetic connectivity, respectively, of these cities to the overall population network. We discuss these results as they challenge the use of cities as replicates of urban eco-evolution, and have implications for conservation and human health in a rapidly growing urban habitat.

随着城市化进程极大地重塑自然景观并在城市中催生全新生境，城市栖息物种的基因流（gene flow）发生改变的潜力随之提升。西部黑寡妇蜘蛛（Western black widow spider, Latrodectus hesperus）是一类与医学相关的城市适应性害虫物种，我们此前已发现其种群遗传特征符合城市化促进基因流的模式，该过程可能由人类介导的运输所导致。本研究针对190万个全基因组单核苷酸多态性位点（SNPs，即Single Nucleotide Polymorphism的复数形式）展开分析，对美国西部10个城市区域与11个非城市区域的大范围地理种群分析，以及涵盖30个城市区域的城市内部精细尺度种群分析进行对比。这套分层数据集使得我们能够检验相关假说，以探究城市化如何在不同空间尺度下对多个城市及其种群的遗传连通性（genetic connectivity）产生影响。结合精细尺度与大范围的分析结果显示，城市内部种群间的遗传分化程度呈现高低两种对比模式：高遗传分化对应这些城市与整体种群网络间较低的遗传连通性，低遗传分化则对应较高的遗传连通性。我们对上述结果展开了讨论，认为该研究结果挑战了将城市作为城市生态-进化（eco-evolution）研究重复单元的通用做法，同时对于快速扩张的城市生境的保护以及人类健康具有重要的启示意义。