Comparative population genetics of Amblyomma maculatum and Amblyomma americanum in the mid-Atlantic United States.

The Gulf Coast tick, Amblyomma maculatum, is undergoing a northward expansion along the United States East Coast, most recently establishing populations in Virginia, Maryland, and Delaware. This expansion has human health implications, as A. maculatum is the primary natural vector of the bacterium Rickettsia parkeri, which causes a spotted fever-type rickettsiosis. Newly established populations of A. maculatum in Virginia tend to have high prevalence of R. parkeri, compared to lower infection rates in the historical range. The factors contributing to high R. parkeri prevalence in Virginia are not known. Investigating connectivity between sites colonized with A. maculatum can help determine whether sites with higher prevalence are isolated or well-connected through migration, thus serving as a source of infected individuals. We characterized 16S rRNA haplotypes of A. maculatum and, for comparison, the congeneric Amblyomma americanum collected from sites where these species co-occur. We then explored connectivity and genetic structure among Virginia populations using pairwise Φ(ST) and AMOVA analyses. Our study identified one recently restored native grassland site with low A. maculatum haplotype diversity and strong evidence of a founder effect, whereas most sites are haplotypically diverse but with no clear genetic structure or connectivity between sites. These findings contrast with high connectivity and a slight mainland/island structure among A. americanum populations. Our results suggest that A. maculatum populations occasionally arise following long-distance drop-offs of few individual ticks in suitable habitat, but no clear migration patterns were observed. The distinct population genetic patterns between species might result from differences in host utilization.