Bites from blacklegged ticks are the cause of most human Lyme disease in the eastern United States. This tick is found throughout the region, whereas most Lyme disease is reported from the Northeast and upper Midwest. Over the past three years, field studies have revealed that this mis-match between the distribution of ticks and disease is the result, in part, of a surprising difference in the behavior of juvenile nymphal ticks. In the North, nymphs looking for a blood meal often emerge from the leaf litter and climb a convenient twig to wait for a passing mouse or bird. This puts a passing human at risk because the nymph can attach to a sock or skirt or exposed ankle and from there find a place to bite. In contrast, Southern nymphs rarely exhibit this climbing behavior because they stay in the leaf litter, and so rarely have an opportunity to attach to a passing human. This behavior appears to be genetically determined, southern ticks continue to behave like southern ticks, even if they have been raised in the North. But what if northern and southern ticks breed with each other (which other researchers have shown is possible) and how will their offspring behave? This study aims to answer that question by raising nymphs from various combinations of parents that either share or differ in their host-seeking behavior. The behavior of these hybrid offspring will be observed in order to learn about the outcomes to be expected if northern and southern ticks begin to mingle in the wild. This research question is important because northern ticks are presently expanding southwards in several states, including Ohio, Virginia, Illinois, and Indiana. Results from this research will be of value to health care providers and the public.
In prior work the PIs utilized a field reciprocal translocation experiment and found that there is a significant genetic basis to the differential host-seeking behavior of nymphs from the northern and southern parts of their range. Overall, the behavioral differences were so pronounced that they appear sufficient to explain the two-orders-of-magnitude difference in Lyme disease incidence between the northern and southeastern US. This award now allows the researchers to address a key question raised by their prior work: what questing behavior will be exhibited once northern and southern tick populations converge and hybridize? And, in what way will these host-seeking traits evolve? The PIs will conduct hybridization experiments between two northern and two southern populations, measure the fecundity of the crosses, and quantify host-seeking behavior of the resulting offspring in field arenas in both Michigan and Tennessee. The research aims to predict how Lyme disease risk will change in the South subsequent to invasion of northern tick populations into states currently occupied by southern populations. Furthermore, this project addresses classic questions regarding the role of genetics and environment on geographic variation of phenotypes and the ability of such traits to evolve.
