3. Hybridization and Range Expansion
Over the last few decades, I. scapularis has continued to establish new populations in counties and states that previously had recorded limited or no activity of I. scapularis (R. J. Eisen et al., 2016), including major expansion north through Canada (Clow et al., 2017; Ogden et al., 2006). These Canadian populations are likely to be offshoots of other Northern U.S. populations from migrating birds (Krakowetz et al., 2011; Scott et al., 2001), contributing to an increased risk of Lyme disease where risk had previously been low. Northern populations are also converging in the Ohio River Valley region, where researchers are seeing significant increases of Lyme disease cases at the border of the Northeastern and Upper Midwestern populations (Bisanzio et al., 2020). Ticks in this region appear to be more evenly mixed genetically between the Northeastern, Upper Midwest, and Central clusters than other populations in our dataset (Fig. 2).
Range expansion is also a major concern in U.S. regions where the area was dominantly inhabited by genetically Southern populations, but now co-exist with Northern populations. Understanding how genes flow between Northern and Southern populations is important for determining how population differences could impact the maintenance and spread of the enzootic cycle of B. burgdorferi s.s. , and consequently, that of Lyme disease. However, directly measuring the vectorial capacity of tick populations is challenging, and it would be more efficient to identify genetic markers in I. scapularis that are associated with high or low human Lyme disease incidence. These markers would then facilitate tracking gene flow within these areas using hybridization zone modeling and next generation sequencing techniques. The loci identified herein provide potential candidate markers for such work.
A potential focal area for this work is the Virginia-North Carolina border which is hypothesized to be situated at the southern spatial expansion front for northern I. scapularis populations (Kelly et al., 2014). The North Carolina populations in our study are at this border and the outer edge of 95% of Lyme disease kernel. However, this region has many other environmental and ecological complexities to consider, such as, the varying Ecological Units (Carpenter et al., 1995) from east to west that could be affecting gene flow across the state (Van Zee et al., 2015). Using the tick mitochondrial 16S rRNA genes, Qiu (2002) found that inland populations in North Carolina have more genetic diversity than more coastal populations potentially due to the elevation changes that bisect the state (Qiu et al., 2002). The Southern coastal populations in our study tended to be less admixed than the two inland North Carolina populations. The effect hybridization of Northern and Southern populations has on tick phenotypes and Lyme disease prevalence in a region could be realized by sampling populations of I. scapularis east to west and north to south in North Carolina.