Discussion:
Reconstructing the invasion pathways for non-native pests represents a
critically important step for the prevention of additional introductions
(Le Roux & Wieczorek, 2009), and can help the implementation of
biosecurity programs for particular pest species and their relatives by
identifying the most likely entry routes (e.g., Maxwell, Vettraino,
Eschen, & Andjic, 2014). Moreover, reconstruction of the invasion
history is crucial for effective biological control, as it enables the
identification of regions from which to evaluate natural enemies (e.g.,
Goolsby et al., 2006). For evolutionary and ecological studies of
factors that influence the probability of establishment of invasive
species and how non-native species adapt to their introduced
environments and ecosystems, (Allendorf & Lundquist, 2003; Dlugosch &
Parker, 2008; Lavergne & Molofsky, 2007), identifying the number of
introductions of an invasive organism is critical (Sakai et al., 2001).
Unfortunately, reconstructing the invasion histories of non-native
organisms can both computationally demanding and/or biologically
untenable due to the combined effects of genetic bottlenecks,
hybridization, and rapid evolution (Buhk & Thielsch, 2015; Ficetola,
Bonin, & Miaud, 2008; McEvoy, Higgs, Coombs, Karçetin, & Starcevich,
2012; Mesgaran et al., 2016; Prentis, Wilson, Dormontt, Richardson, &
Lowe, 2008). Using a combination of Bayesian clustering and approximate
Bayesian computation methods, we find that invasive populations of
winter moth in Nova Scotia, British Columbia, and the northeastern
United States were all introduced separately from Central Europe, and
that the invasive population in Oregon was introduced from Western
Europe (Figures 3 and 4). Distance analyses of the coefficient of
membership assignments from our Bayesian clustering analyses, suggest
that the invasive population in Nova Scotia is most closely related to a
population of winter moth in Orleans, France, and that the invasive
population in British Columbia is most closely related to a population
of winter moth in Uggvallen, Sweden (Figure 3), representing potential
source localities for both Canadian populations.
The relationships of the American populations were less clear based on
distance analyses, however, with the Oregon population being closely
related to a large number of populations from the British Isles and
northern Fennoscandia, and the northeastern United States population
being unrelated to any sampled European population. In a recent study of
winter moth in Fennoscandia, we determined that the population there was
introduced to the region ~2,000 ya from the British
Isles, either via dispersal across the North Sea or by human-mediated
dispersal (Andersen et al. In Press). Therefore, without finer-scale
genomic analyses (e.g., genotype-by-sequencing), we will unlikely be
able to determine which of these two locations (British Isles or
northern Fennoscandia) are the source of the Oregon population. The
northeastern United States population, in contrast to all other sampled
invasive populations, was clearly distinct from other winter moth
populations, including the invasive population in Nova Scotia that can
be found only a short distance away across the Bay of Fundy. While it is
possible that we have yet to sample the source of this invasive
population, given our extensive sampling of the native distribution of
winter moth, we believe this is unlikely. For example, the only regions
that we did not sample from which winter moth has previously been
reported are Japan, Taiwan, and the Russian far-east (Troubridge &
Fitzpatrick, 1993), and in those locations, records of winter moth are
most likely misidentifications of different species that are native to
those regions (Nakajima, 1991). In addition, our inclusion of an
unsampled “ghost” population in our DiyABC analyses to
represent a possible extra-European source was not supported in any
tournament (P ≤ 0.01 in all analyses). Lastly, as part of ongoing
efforts to study the evolution of the genus Operophtera , we have
worked with collaborators in both the Russian far-east and in Japan to
collect samples using traps baited with the winter moth sex-pheromone,
and based on preliminary DNA-barcoding results all samples that were
collected in these regions have been native Operophtera species
(NPH unpublished data). Therefore, we believe that the genetic
distinctness of the northeastern United States population is likely the
result of a strong genetic bottleneck and/or rapid-local adaptation
following its introduction from Central Europe.