Genetic and Landscape Connectivity of Blacklegged Ticks During Range Expansion in Select States of the Midwestern USA.

Since the 1970s, the Midwestern USA has experienced an expansion of blacklegged ticks (Ixodes scapularis), the primary vector of Lyme disease caused by Borrelia burgdorferi, leading to increased Lyme disease incidence. Public health surveillance indicates that Northwestern Wisconsin has served as refugia for these ticks, seeding an expansion into neighboring states such as Michigan. However, the process of re-emergence and invasion remains unclear. To improve tick management, we examine whether environmental variables in the Midwestern (eastern North Central) region have constrained tick dispersal and whether connectivity corridors can be identified. By developing fine-scale spatial population genomic data, our analyses reveal genetically diverse populations in Wisconsin, with northern populations contributing to recent expansions within the state. We identify several east-west gene flow corridors facilitating tick dispersal in Wisconsin. An independent dispersal network exists along Wisconsin's Mississippi River, extending southwards. In contrast, Michigan populations exhibit sharp genetic divergence from Wisconsin and Indiana populations, with low genetic diversity and high in-state gene flow. We also identify high landscape connectivity in the region connecting the Michigan Peninsulas and significant gene flow at the landmass near southern Lake Michigan. Geographical isolation, as well as landscapes with low soil humidity during summer and high human disturbance, were found to limit gene flow across the region, although these effects were minor. Management of blacklegged ticks in the region can be enhanced by recognizing that landscape connectivity has influenced the dispersal of distinct genetic populations, and targeted interventions in seemingly less tick-favorable landscapes may help mitigate the spread.