3.1 | Population structure
Strong genetic structure was uncovered among the R. flavipesindividuals from fastSTRUCTURE, with K = 4 best explaining the structure
in the data (Fig. 1). At this value of K, most individuals in the
dataset (57.2%) were clearly assigned to one of the four clusters
(assignment probability higher than 99%) (73.3% of individuals were
assigned to a unique cluster probability higher than 80%). However, the
strong genetic structure uncovered among individuals in the native range
was inconsistent with their geographic origin, as neighboring samples
often exhibited completely different assignment profiles (Fig. 1). In
the French introduced range, most samples could be assigned to the same
cluster, although some samples from the Paris region had a mixed
assignment; a similar mixed assignment was found for the lone German
sample. A comparable pattern was observed in the Chilean introduced
range, with most samples displaying fixed assignments and only a few
with mixed assignments. Although most individuals were assigned to a
unique genetic group within each introduced population (France, Chile
and Canada), the three introduced populations were separately assigned
to three different genetic groups and did not segregate into a single
‘introduced’ cluster. Because the genetic clustering of the
native range did not consistently align with geographic origin,
inferring a source population for each introduced population becomes
difficult. For example, most samples from Chile were assigned to the
same cluster as samples from New York, Wisconsin and Texas. Similarly,
although the introduced population in France shared its strongest tie to
the native range with Arkansas, France also had ties with Louisiana,
Missouri and even one sample in South Carolina. The origin of the
samples in Canada was even more complicated, as the genetic cluster
present in this population was spread across most native localities.
Similar results to that of fastSTRUCTURE were uncovered using the PCA
and DAPC approach (Figure 2). The PCA indicated strong differentiation
across R. flavipes samples, as they broadly segregated along the
two axes. For most localities, genetic clustering was not correlated
with geography, as samples from a given locality did not always cluster
together. Likewise, no genetic similarity was observed between
geographically neighboring localities. Interestingly, such a pattern was
also found to a lesser extent in the introduced populations, especially
between France and Chile (only a single sample was available from
Germany and Uruguay, and just two from the Bahamas). In France, most of
the samples segregated together, except for six individuals clustering
separately from the rest of the main population. These samples
correspond to the individuals from the Paris region exhibiting a mixed
fastSTRUCTURE assignment. A similar pattern was observed for the samples
from Chile, with three samples clustering apart from the main Chilean
population. The find.clusters algorithm found the best support
for four genetic clusters in the dataset (Figure 2). Notably, the
introduced localities of R. flavipes did not cluster together;
instead, the different introduced populations were spread across the
four different DAPC clusters, with some even split between two clusters
(Chile and France). Remarkably, a similar pattern was observed from
localities within the native range, with samples from a given locality
clustering into two (e.g., Texas, Mississippi, Wisconsin) or even
three (Louisiana) distinct DAPC clusters.
The co-ancestry matrix highlighted similar patterns when clustering
individuals based on their level of relatedness (Figure 3). Using
fineRADstructure, all samples from a given locality were not more
related to one another than they were to samples from another locality
(Figure 3). This result is indicative of an absence of geographic
structure in the native range, as most localities were disjunct on the
co-ancestry matrix. Notably, the same pattern was observed for the
introduced populations, with clustering observed in two (Canada) or
three (France and Chile) distinct co-ancestry groups.