Introduction:
Hybridization is a driver of speciation and evolutionary trajectories
across the tree of life (Allendorf, Leary, Spruell, & Wenburg, 2001;
Costedoat, Pech, Chappaz, & Gilles, 2007; Harrison & Larson, 2014;
Mallet, 2005). While numerous pre- and post-zygotic barriers exist in
most natural ecosystems to reduce genetic exchanges between species,
human-mediated disturbance and climate change have led to increased
hybridization rates across a diversity of taxonomic groups (Gomez,
Gonzalez-Megias, Lorite, Abdelaziz, & Perfectti, 2015; Hegarty, 2012;
Larson, Tinghitella, & Taylor, 2019). The accidental introduction of
non-native organisms to novel habitats has further increased these rates
by uniting previously disjunct species or genetically distinct
populations (Chown et al., 2015; Havill et al., 2012; Havill et al.,
2021; Michaelides, While, Bell, & Uller, 2013). In addition to
illuminating factors that may be important in invasion ecology, studying
the real-time formation of hybrid zones between native and non-native
species may provide a type of natural laboratory, providing important
insights into how other well-established hybrid zones may have formed
and settled over evolutionary timescales. As such, recent work has
highlighted the importance of studying newly-formed hybrid zones for
understanding speciation and the preservation of species boundaries
(Johannesson, Le Moan, Perini, & Andre, 2020; Larson et al., 2019).
These natural laboratories are particularly important because most
documented hybrid zones have likely existed for thousands of years and
formed following the movement of species in response to long-term
processes such as changing climates during the Quaternary climatic
oscillations (e.g., Ryan et al., 2018; Ryan et al., 2017; Scriber, 2011;
Taylor, Larson, & Harrison, 2015). Natural hybrid zones frequently have
a clinal structure, with a narrow, linear geographic zone of admixture
where phenotypic and genetic states change across a gradient between
parent species (Barton & Hewitt, 1985; Endler, 1977). In contrast, most
documented hybrid zones created in contemporary settings between
introduced and native species have a mosaic structure (see Harrison &
Rand, 1989), with zones of genetic exchange spread across the landscape
in a patchy and non-linear fashion (e.g. Cordeiro et al., 2020; Havill
et al., 2012). Therefore, additional examples of newly formed clinal
hybrid zones are needed to better understand the evolutionary and
ecological processes that shape these temporally and spatially dynamic
regions of secondary contact.
Species of moths and butterflies (Insecta: Lepidoptera) have provided
some of the most stunning examples of the diversity of interactions
resulting from hybridization (e.g., Ipekdal, Burban, Saune, Battisti, &
Kerdelhue, 2020; Lucek, Butlin, & Patsiou, 2020; Ryan et al., 2018;
Ryan et al., 2017; Scriber, 2011). Here we explore the formation of a
hybrid zone in northeastern North America between the introduced
European winter moth, Operophtera brumata L. (Lepidoptera:
Geometridae) and the native Bruce spanworm, O. bruceata Hulst.
Winter moth is native to western Eurasia and North Africa (Ferguson,
1978) and originally became established in North America in Nova Scotia
in the 1930s, where it was identified as a major pest in apple orchards
and oak-dominated hardwood forests (Embree, 1966, 1967). Subsequently,
populations were identified in Oregon as a pest in hazelnut (filbert)
orchards in the 1950s (Kimberling, Miller, & Penrose, 1986), British
Columbia as a pest in apple orchards and of urban trees in the 1970s
(Gillespie, Wratten, Cruickshank, Wiseman, & Gibbs, 1978), and most
recently in the northeastern United States (hereafter, the
“Northeast”) as a pest of blueberries, cranberries, and many native
deciduous trees in the 1990’s (Elkinton et al., 2010; Elkinton,
Liebhold, Boettner, & Sremac, 2014). Each of these regions were likely
the result of independent invasions from Europe (Andersen, Havill,
Caccone, & Elkinton, 2021), and while successful biological control
programs have reduced the abundance and economic impacts of this
important pest in each invaded region (Elkinton, Boettner, Liebhold, &
Gwiazdowski, 2015; Elkinton, Boettner, & Broadley, 2021; Kimberling et
al., 1986; Roland & Embree, 1995), populations of winter moth continue
to persist at low densities in each location. Previous work in this
system has shown that winter moth and Bruce spanworm hybridize readily
in the field (Andersen et al., 2019; Elkinton et al., 2010, 2014; Havill
et al., 2017). Additionally, in the Northeast it has been documented
that the proportions of individuals of winter moth versus Bruce spanworm
can be modeled using logistic regression, with populations proximate to
Boston, Massachusetts being nearly 100% winter moth, and populations in
western Massachusetts being nearly 100% Bruce spanworm (Elkinton et
al., 2014). This gradient in winter moth and Bruce spanworm population
densities in the Northeast therefore raises the possibility that a
clinal hybrid zone may exist in this region, making it one of the first
documented cases of this type of hybrid zone between an introduced and a
native species.
We explored the spatial and temporal dynamics of the hybrid zone between
the invasive winter moth and native Bruce spanworm by collecting moths
with pheromone traps along two transects that crossed the leading edge
of winter moth spread in the Northeast region. One of these transects
was located along Route 2 in Massachusetts (hereafter the
“Massachusetts transect”) and was sampled over a 12-year period (from
2007 to 2018) where a gradient across decreasing extreme cold winter
temperatures has been hypothesized to limit the distribution of winter
moth (Elkinton, Lance, Boettner, Khrimian, & Leva, 2011). The second
transect is located along the coast of southern Connecticut following
Route 1 (hereafter the “Connecticut transect”) and was sampled over a
3-year period (2016-2018). This second transect was added so that we
could compare the role of winter temperatures on the geographic location
of the hybrid zone as temperatures along the Connecticut transect are
milder than at any point along the Massachusetts transect, and
geographic settling would therefore be independent of low winter
temperatures. With these data, we explore: 1) the structure and movement
of the hybrid zone in the Northeast, 2) changes in the rate of
hybridization over time, and 3) the impacts of environmental gradients
and population densities in the regulation of the hybrid zone.