Phylogeography of the blacklegged tick (Ixodes scapularis) throughout the United States identifies candidate loci for differences in vectorial capacity.

The blacklegged tick, (Ixodes scapularis Say 1821) is a vector of Borrelia burgdorferi sensu stricto (s.s.) Johnson et al. 1984, the causative bacterial agent of Lyme disease, part of a slow-moving epidemic of Lyme borreliosis spreading across the northern hemisphere. Well-known geographic differences in the vectorial capacity of these ticks are associated with genetic variation. Despite the need for detailed genetic information in this disease system, previous phylogeographic studies of these ticks have been restricted to relatively few populations or few genetic loci. Here we present the most comprehensive phylogeographic study of genome-wide markers in I. scapularis, conducted by using 3RAD and surveying 353 ticks from 33 counties throughout the species' range. We found limited genetic variation among populations from the Northeast and Upper Midwest, where Lyme disease is most common, and higher genetic variation among populations from the South. We identify five spatially-associated genetic clusters of I. scapularis. In regions where Lyme disease is increasing in frequency, the I. scapularis populations genetically group with ticks from historically highly Lyme-endemic regions. Finally, we identify ten variable DNA sites that contribute the most to population differentiation. These variable sites cluster on one of the chromosome-scale scaffolds for I. scapularis and are within identified genes. Our findings illuminate the need for additional research to identify loci causing variation in the vectorial capacity of I. scapularis and where additional tick sampling would be most valuable to further understand disease trends caused by pathogens transmitted by I. scapularis.