Little population structure within the American dog tick, Dermacentor variabilis, across North America

The American dog tick, Dermacentor variabilis, is one of the most important vectors of bacterial pathogens to wildlife, domestic animals, and humans in North America. Although this tick is widely distributed in the U.S. and Canada, knowledge of its range-wide phylogeographic patterns remains incomplete. To resolve this, we carried out a phylogenetic analysis using DNA sequences from the mitochondrial cytochrome oxidase I (COI) gene. Tick samples came from two main sources: 1) existing archives from collaborators, and 2) new collections from 2012-2013. We sequenced a 691 bp fragment of the COI gene from geographically diverse D. variablis (n=401) collected throughout the U.S. and Canada.  Maximum parsimony analysis of COI sequences recovered two major subgroups within the species with distinct geographic distributions: one from the eastern U.S./Canada and the other from the west coast of the U.S. Interestingly, we did not observe any genetic subdivisions within these two major groups that also correlated with geography. This suggests high levels of genetic admixture among widely separated geographic localities, which may be due to range expansion or long-distance movements mediated by humans. We also screened a larger sample set (n=1053) for the presence of important bacterial species using a 16s rRNA sequencing approach and two species-specific qPCR assays. We found Francisella-like endosymbionts, Coxiella spp., and Rickettsia spp. among our samples.  This work has implications for understanding the dispersal and evolutionary ecology of this tick and associated vector-borne diseases, as detailed knowledge of vector population structure is vital for understanding pathogen transmission.