Neutral and Adaptive Population Structure
In the S. umbilicalis neutral marker dataset neither DAPC nor
TESS3 were able to discern any genetic clusters using the
cross-validation score, supporting K=1. Although several estimates of
pairwise FST were significantly greater than zero (Fig.
2A), the uniformly low values across all population comparisons (Table
S7) support the DAPC/TESS3 results indicating no substantial genetic
structure. For the outlier dataset, both methods identified four as the
optimal number of clusters (Fig. 2B-C). At K=4 both methods largely
agreed on the geographic distribution of clusters: 1) sites in Wales, 2)
Scotland and north-west England, 3) Irish Sea coasts of Ireland and
Northern Ireland, and 4) south and south-eastern Ireland, with DAPC
suggesting larger proportions of admixture in Irish populations. These
results agree with estimates of FST (Fig. 2A; Table S7),
suggesting that the largest degree of differentiation in outlier loci is
between the Welsh populations and all other populations.
In the N. lapillus neutral marker dataset, DAPC identified five
as the optimal number of genetic clusters (Fig. 3D), whereas TESS3
suggested 11 (Fig. 3B). At K=5, the genetic groups largely corresponded
to the spatial distribution of sites comprising: 1) south-western
Scotland, 2) Northern Ireland, 3) south and south-eastern Ireland, 4)
north-western England and north Wales, and 5) west Wales and eastern
Ireland (Fig. 3C). The exception of these spatial groupings is Barry, a
south Wales site, which clustered with sites in north Wales and
north-western England. DAPC identified limited unidirectional admixture
from the Welsh/English cluster to the Irish Sea and south-eastern
Ireland. At the higher level of structure, K=11, most clusters
corresponded to a single site, except for two clusters comprising both
Scottish sites and both sites in north Wales (Fig. 3B). In the outlier
dataset, TESS3 and DAPC suggested eight and seven genetic clusters,
respectively, and largely agreed on the distribution of these genetic
clusters (Fig. 3C, E). Estimates of FST suggest that
Welsh populations are generally the most differentiated in both neutral
and outlier datasets (Fig. 3A; Table S9).
Although we detected a significant linear relationship between shortest
marine distance and neutral FST in both species (Fig.
4), Mantel tests suggested a marginally significant effect of IBD in theS. umbilicalis dataset only (N. lapillus : Spearman’s
r=0.1485, p=0.153; S. umbilicalis : Spearman’s r=0.2346, p=0.048).