Northern Fennoscandia via the British Isles: evidence for a novel post-glacial recolonization route by winter moth (Operophtera brumata)

Four times out of Europe: Serial invasions of the winter moth, Operophtera brumata, to North America

“…region -we determined that the population there was introduced to the region ~2000 years ago from the British Isles, either via dispersal across the North Sea or by human-mediated dispersal (Andersen et al, 2021). Therefore, without finer-scale genomic analyses (e.g., genotype-by-sequencing), it is unlikely that we will be able to determine which of these two locations (British Isles or northern Fennoscandia) are the source of the Oregon population.…”

“…The relationships of the American populations were less clear based on distance analyses; however, as the Oregon population was closely related to a large number of populations from the British Isles and northern Fennoscandia, while the northeastern United States population was unrelated to any sampled European population. In a recent study of winter moth in Fennoscandia – where winter moth exists at outbreak densities in much of the region – we determined that the population there was introduced to the region ~2000 years ago from the British Isles, either via dispersal across the North Sea or by human‐mediated dispersal (Andersen et al, 2021). Therefore, without finer‐scale genomic analyses (e.g., genotype‐by‐sequencing), it is unlikely that we will be able to determine which of these two locations (British Isles or northern Fennoscandia) are the source of the Oregon population.…”

“…After collection, adult moths were placed in glassine envelopes (Uline Corporation) and stored at either –20 or –80°C, and larval caterpillars were placed in 95% ethanol and stored at –20°C. For many of the moths included in our analysis (1361 out of 1839), collection and genotype information have been previously reported by us in studies of winter moth population structure in the native or introduced ranges (Andersen et al, 2017; Andersen, Havill, Broadley, et al, 2019; Andersen, Havill, Mannai, et al, 2019; Andersen et al, 2021; see Table ).…”

“…To determine whether populations of North American winter moth in distinct geographic regions were the result of a single introduction to Nova Scotia (the first introduced region recorded in North America) that was then subsequently spread to additional locations in North America (i.e., following a stepping stone model), or whether each invasive population represents a novel introduction (i.e., a serial introduction model), or some combination of these, we compared the relatedness of each invasive population to each other and to the "Eastern European", "Central European", and "Western European" winter moth genetic clusters previously reported in Andersen et al (2017Andersen et al ( , 2021 using approximate Bayesian computation (ABC), as implemented in the software program diyabc v.2.1.0 (Cornuet et al, 2008). For these analyses, 30 individuals were selected randomly from each of the three European clusters and from each of the four invasive regions.…”

“…Under each scenario we included multiple parameters to allow for changes in population sizes, following splitting/merging events, include genetic bottlenecks during the introduction of each invasive population (modeled following Lombaert et al, 2014), utilize default mutation model parameters, a minimum mean mutation rate of 1 × 10 −5 , and the maximum values for the mean and individual locus coefficient P's were both set to 1.0. As per Andersen et al (2017Andersen et al ( , 2021, we removed four loci with especially large allelic ranges (02339, 00925, 02191, and 12042) to improve the shape of the cloud of simulated data sets. We calculated three, one sample summary statistics (mean number of alleles, mean genetic diversity, and mean size variance) and three two sample summary statistics (F ST , classification index, and [dμ] 2 distance).…”