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.