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

Abstract The blacklegged tick (Ixodes scapularis(Journal of the Academy of Natural Sciences of Philadelphia, 1821,2, 59)) is a vector ofBorrelia burgdorferisensu stricto (s.s.) (International Journal of Systematic Bacteriology, 1984,34, 496), the causative bacterial agent of Lyme disease, part of a slow‐moving epidemic of Lyme borreliosis spreading across the northern hemisphere. Well‐known geographical 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 phylogeographical studies of these ticks have been restricted to relatively few populations or few genetic loci. Here we present the most comprehensive phylogeographical study of genome‐wide markers inI. scapularis, conducted by using 3RAD (triple‐enzyme restriction‐site associated sequencing) 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 ofI. scapularis. In regions where Lyme disease is increasing in frequency, theI. scapularispopulations genetically group with ticks from historically highly Lyme‐endemic regions. Finally, we identify 10 variable DNA sites that contribute the most to population differentiation. These variable sites cluster on one of the chromosome‐scale scaffolds forI. scapularisand are within identified genes. Our findings illuminate the need for additional research to identify loci causing variation in the vectorial capacity ofI. scapularisand where additional tick sampling would be most valuable to further understand disease trends caused by pathogens transmitted byI. scapularis.