Phylogeography of the blacklegged tick Ixodes scapularis throughout the
United States identifies candidate loci for differences in vectorial
The blacklegged tick, Ixodes scapularis, is a vector of Borrelia
burgdorferi sensu stricto (s.s.), the causative agent of Lyme disease,
part of a slow-moving epidemic of Lyme borreliosis spreading across the
northern hemisphere. There are well-known geographic differences in the
vectorial capacity of these ticks 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 genetic loci. Here we
present the most comprehensive phylogeographic study of I. scapularis
conducted by using 3RAD and surveying 353 ticks from 33 counties
throughout the range of I. scapularis. 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 four genetic clusters of I.
scapularis that are consistent with four major geographic regions, plus
a distinct Central Florida group. 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.