Introduction
Determining the appropriate delineation of populations and species is
critical to understanding many biological systems. Several different
species concepts are being used depending on the characteristics of the
organisms of interest and the lens of researchers studying them (de
Queiroz, 2005). Accurately identifying the differences across closely
related species, clades, and populations is beneficial to assessing the
existing diversity of the group in question (Sokal & Crovello, 1970).
Vectors offer opportunities to study complex ecological interactions
among biological groups with varying levels of differentiation and
co-varying levels of association. In vector biology, these differences
can significantly change the potential risk for vector-borne pathogen
transmission within different environments (Arsnoe et al., 2019; Dolan
et al., 1998; L. Eisen et al., 2017), and the organismal classification
can influence how public health officials track disease risk in a region
(Lane et al., 1991; Oliver et al., 1993).
The blacklegged tick, Ixodes scapularis , is a vector in the
eastern and midwestern U.S. for at least seven human pathogens includingAnaplasma phagocytophilum, Borrelia mayonii, Borrelia miyamotoi,
Ehrlichia muris eauclarensis, Babesia microti , Powassan virus, and most
notably, Borrelia burgdorferi sensu stricto (s.s.) , a causative
agent of Lyme disease (R. J. Eisen & Eisen, 2018). Lyme disease is a
nationally notifiable condition in the U.S. (CDC, 1990) and cases have
been continuously increasing for years (Bacon et al., 2008; Division of
Vector-Borne Diseases, 2019; Rosenberg et al., 2018). More than 95% of
human Lyme disease cases occur in the northeast and the upper Midwest
regions of the U.S. (Division of Vector-Borne Diseases, 2019; Kuehn,
2013), with a low incidence of cases occurring throughout the rest of
the U.S. (Figure 1). While the majority of human Lyme disease cases in
the U.S. are due to transmission of B. burgdorferi s.s. byI. scapularis, other Ixodes species have been shown to be
competent vectors (Couper et al., 2020; Fleshman et al., 2021).
The taxonomy of I. scapularis has been debated for many years.
Currently, I. scapularis is subclassified to the Ixodes
ricinus species complex, notably containing the main vectors of
Lyme-causing Borrelia spirochetes. From 1979 (Krinsky, 1979)
until 1996 (Oliver et al, 1993), there were three tick species from theI. ricinus -species complex located within the U.S., that were
associated with transmitting B. burgdorferi s.s. : Ixodes
pacificus in the West, I. scapularis in the Southeast, andI. dammini in the Northeast (Lane et al., 1991). However,I. scapularis were publicly believed to not be a large threat as
they were thought to rarely bite humans (Oliver, 1996). Due to the
geographical closeness and the intermediate characteristics found in
nature of I. dammini and I. scapularis, the validity of
the delination between the two was questioned (Spielman et al., 1979;
Oliver et al., 1993). Laboratory mating experiments determined them to
be conspecific species and I. dammini was synonymized withI. scapularis (Oliver et al., 1993). Once I. dammini ,
which was previously considered the main threat, was no longer
considered a valid species, the concern for and surveillance of Lyme
disease increased because the main vector, I. scapularis , is
spread throughout the eastern U.S. (Oliver et al., 1993; Sanders, 1998).
This new perception of I. scapularis led to questions on how
populations of this species differ, especially when it came to Lyme
disease prevalence in a region. Subsequent genetic studies redefinedI. scapularis into two distinct clades, delineating Northern
populations (American clade) from Southern populations (Southern clade)
(Norris et al., 1996).
Researchers have been using the American and Southern clades as a
general way to classify Ixodes scapularis populations while the
geographic range of the species has been expanding to the north and
west. Populations have been documented in almost every state east of the
Rocky Mountains while newly established populations are filling in
previously uninhabited counties throughout the known range and can now
even be found as far north as Québec (R. J. Eisen et al., 2016; Fleshman
et al., 2021; Khatchikian et al., 2015; Ripoche et al., 2022). This
continual range expansion is increasing the potential risk for Lyme
disease throughout North America (Kelly et al., 2014; Kugeler et al.,
2015; Leighton et al., 2012; Ogden et al., 2009). The Lyme endemic
region is also expanding outward from the historical locations in the
Northeast and upper Midwest, with some counties south of Virginia
reporting increases in I. scapularis abundance (R. J. Eisen et
al., 2016; Hickling et al., 2018) concomitant with increases in Lyme
disease cases in humans over the past two decades (Division of
Vector-Borne Diseases, 2019). States within the Ohio River Valley region
are experiencing some of the highest rates of Lyme disease case spread
in the U.S. For example, Lyme disease cases in Ohio have increased
10-fold over the past decade, likely due to the range convergence of the
Northeastern and Upper Midwestern I. scapularis populations
(Bisanzio et al., 2020).
The presence of I. scapularis alone is not enough to predict how
rapidly Lyme disease risk can increase in a region because Lyme disease
ecology is complex and involves multiple intersecting factors within the
tick life cycle, such as dominant tick blood meal hosts and reservoir
vertebrate hosts of B. burgdorferi (Bisanzio et al., 2020; Burtis
et al., 2016; Gardner et al., 2020; Ogden et al., 2013). These
intersecting factors all contribute to the vectorial capacity ofI. scapularis for B. burgdorferi , defined as all
“vector-related variables affecting stability of pathogen
transmission” (Spielman et al., 1984). In addition to variation in host
preference, a key determinant of pathogen transmission by I.
scapularis is likely in the host-seeking behaviors themselves, such as
time spent host-seeking, or the height to which ticks will climb for
host-seeking. Population level variation of host-seeking could be due to
changes in humidity, because ticks are extremely sensitive to
desiccation, and host community composition (i.e., mice are the most
common host of tick larvae in the north, whereas lizards are important
hosts in the south (Apperson et al., 1993; Ostfeld & Keesing, 2000)).
However, previous research has also shown that genetic factors are
likely contributing to a tick’s ability to acquire and transmit
pathogens and genetics could even play a role in host-seeking behavior
differences (Arsnoe et al., 2015; Ginsberg et al., 2017; Ginsberg et
al., 2014). Thus, the genetic composition of the expanding I.
scapularis populations merits consideration.
While there has yet to be a study directly linking tick host-seeking
behaviors to genetic variants, underlying genetic mechanisms driving
population-level differences in I. scapularis survivability and
host-seeking behaviors have been hypothesized. The consistency of these
behaviors within Northern and Southern I. scapularis populations
when exposed to varying environments suggests that host-seeking behavior
is driven in part by genetics, and not solely from environmental
pressures (Arsnoe et al., 2015; Tietjen et al., 2020). Identifying the
populations that are leading the range expansion fronts, like those in
the Ohio River Valley and the Virginia-North Carolina border, will
provide public health officials with an informed view of tick-borne
disease risk in currently non-Lyme endemic areas experiencing invasion
(or possibly re-invasion) by blacklegged ticks. However, due to the
difficult and time-consuming efforts needed to directly measure
vectorial capacity of ticks, it is far more efficient to identify
genetic markers that are associated with tick populations that have high
or low human Lyme disease prevalence. If the markers are sufficiently
dense and spread throughout the genome, then it may also be possible to
identify loci associated with the traits of interest (Anandan et al.,
2022; Aubry et al., 2020; Dupuis & Siegmund, 1999; Rahman et al.,
2021).
Research to increase our understanding of the population genetics ofI. scapularis has occurred for many decades. The main limitation
so far has been that the studies either do not cover a wide portion of
the genome or the wider geographic range of the species. They also
cannot be easily compiled to accomplish both aspects as the targets and
methodologies are non-compatible. Nonetheless, they have given insight
into how populations of I. scapularis vary genetically,
concluding there is significant genetic differentiation across the
landscape, especially when comparing northern and southern populations
(Gulia-Nuss et al., 2016; Ludwig, 2015; Norris et al., 1996; Van Zee et
al., 2015). A study covering a large portion of the genome and known
range of I. scapularis populations is needed to characterize the
extent of differentiation within this species, as well as advancing an
understanding that links phenotypes to genotypes.
Here, we investigate genome-wide variation within and among populations
of I. scapularis grouped into four geographic regions that
approximate major divisions described or hypothesized in previous
research: Southeastern Atlantic, Southern Gulf, Upper Midwest, and
Northeastern U.S. We aim to provide a basis for future research in
uncovering how genetic variation among I. scapularis populations
is driving observed phenotypic differences, thus driving Lyme disease
prevalence in the U.S. Understanding the genotypic variation within and
among I. scapularis populations, especially those at the fronts
of range expansion and phenotypic change, is essential to understanding
the ecology of these ticks and pathogens, which can mitigate Lyme
disease risks and serve as a model system for disease ecology.