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).