Hybrid origins
Hybridization has long been known to play an important role in the
evolutionary trajectories of species and populations (Allendorf, Leary,
Spruell, & Wenburg, 2001), and the promotion of the establishment of
invasive species (e.g., Allendorf & Lundquist, 2003; Benvenuto,
Cheyppe-Buchmann, Bermond, Ris, & Xavier, 2012; Mesgaran et al., 2016;
Sakai et al., 2001). Soon after the discovery of an invasive population
of winter moth in the northeastern United States, hybridization between
the non-native winter moth and the native Bruce spanworm was
demonstrated by sequencing the G6PD nuclear gene (Elkinton et al., 2010;
2014). Subsequent studies have found that multigenerational
hybridization is occurring between these two species in the northeastern
United States (Havill et al., 2017), and that hybridization between the
two species has occurred in all of the locations that winter moth has
established (Andersen et al., 2019a). In a recent study along a transect
running west (primarily Bruce spanworm) to east (primarily winter moth)
in Massachusetts with traps spaced approximately 10 km apart,
~1% of surveyed individuals were hybrids though in a
distinct hybrid zone between the two species hybridization rates of over
10% were reported at several locations (Griffin, Chandler, Andersen,
Havill, & Elkinton, In Press). Therefore, we were surprised that only
four individuals among our samples were classified as hybrids,
suggesting that hybridization has not played a role in the establishment
of winter moth populations, however, this result could be an artifact of
our analyses given that only 12 of the sampled 24 loci have been shown
to consistently co-amplify between the two species (Havill et al.,
2017). Alternatively, given that the only hybrid individuals were
detected in Oregon, a region from which winter moth has virtually
disappeared after its initial establishment, it is also possible that
some type of hybridization to extinction may be occurring in this
system. This possibility needs to be examined further, perhaps with
genome scans to look for introgression between the species, as it could
present an example of hybridization between native and non-native
species having a positive impact, counter to the “hybridization to
extinction” paradigm that puts the overwhelming emphasis on risks to
native species (Rhymer & Simberloff, 1996).