Lack of host-associated differentiation in Ixodes scapularis using population genetics

The blacklegged tick, Ixodes scapularis (Say) (Acari: Ixodidae), is a vector of pathogens that cause diseases including anaplasmosis, babesiosis, and Lyme disease. It is a generalist vector feeding on a wide variety of vertebrate hosts. The transmission cycles of these pathogens are affected by the tick's host selection because host species differ in their ability to clear, maintain, or amplify these pathogens. Several generalist parasites exhibit host-associated differentiation (HAD), a process that results in genetically distinct populations associated with different host species. Knowing whether I. scapularis exhibits HAD is important to better understand the ecology of these diseases. To test for HAD in I. scapularis, whole-genome resequencing was performed on ticks collected directly from six hosts in the southern United States (i.e., cotton mice [Cricetidae: Peromyscus gossypinus Le Conte], opossum [Didelphidae: Didelphis virginiana Kerr], raccoon [Procyonidae: Procyon lotor L.], gray fox [Canidae: Urocyon cinereoargenteus Schreber], wild boar or feral hog [Suidae: Sus scrofa L.], and white-tailed deer [Cervidae: Odocoileus virginianus Zimmermann]). For the hosts tested, HAD was not found. However, it could not be ruled out if HAD is occurring for ticks associated with lizards in this study. The results of this study contrast with findings from another host generalist, Dermacentor variabilis (Say), that does exhibit HAD. Our results suggest that these two tick species differ in their propensity for HAD. One explanation could be that I. scapularis can mate off-host, maintaining panmixia, whereas D. variabilis mate only on the host. The lack of HAD in I. scapularis should be confirmed by sampling ticks from lizard and rodent hosts. These results will inform control efforts and the use of area-wide tick control strategies.