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

Abstract

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 genome-wide SNPs, we contrast broad-scale geographical analyses of 10 urban and 11 non-urban locales with fine-scale within-city analyses including 30 urban locales across the western USA. 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.

Keywords: genetic connectivity; population genetics; urban pest; urbanization.

Figure 1.

(a) Geographical distribution of the broad-scale sampled locales of the western black widow spider across the western USA (see electronic supplementary material, table S2). Highlighted locales in blue and yellow reflect urban and non-urban samples, respectively. Fine-scale sampled locales are shown for (b) Albuquerque, (c) Las Vegas and (d) Phoenix (see electronic supplementary material, table S1 for sampling locales), with circles for urban and diamonds for non-urban locales. Colour scale represents the percent impervious surface. (Online version in colour.)

Figure 2.

PC1 and PC2 biplots of individual genotypes are shown for fine-scale sampled locales within (a) Albuquerque, (b) Las Vegas and (c) Phoenix urban areas. The left and right panels reflect urban samples highlighted (colour-scheme) and non-urban samples highlighted (yellow), respectively (see electronic supplementary material, table S1). (Online version in colour.)

Figure 3.

Social network popgraph analysis among urban (blue) and non-urban (yellow) locales for the broad-scale sample (figure 1), as well as the fine-scale sample from Albuquerque (black), Las Vegas (light grey) and Phoenix (dark grey) cities (see electronic supplementary material, tables S1 and S2). The size of each node reflects the locale-specific genetic variance, and the length of the edges is proportional to the conditional genetic distance (cGD; see Methods) between locales. (Online version in colour.)