Results

Population structure

The admixture analysis conducted in NGSadmix showed that European sardines seem to be part of at least three structured populations (Figure 1B). When setting the number of expected clusters to 2 (k=2; Figure 1B, top), one of the clusters is prevalent in the Center region, while the other is more frequent in both Western and Eastern regions, as well as the Canary Islands. Individuals with admixed ancestry from these two clusters were observed at all sampling sites, except at Madeira. For k=3 (Figure 1B, bottom), one of these clusters (West-East-Canaries) splits into two: one frequent in the Mediterranean and Canaries and the other in the West (Madeira and Azores). Admixed ancestry between the three main clusters was observed in individuals from the Central region, Eastern Iberia, South of France, Tunisia and from the Aegean Sea. Canaries’ individuals show some admixed ancestry with the Western cluster; and two individuals from The Adriatic share some ancestry from the Western cluster. The proportion of individuals with admixed ancestry is lower in populations located in the extremes of the distribution: Madeira, Azores in the West, Aegean and Adriatic Sea in the East as well as Bretagne and Bay of Biscay S in the central group.
The organization in three separate clusters can also be observed in the principal component analysis (PCA; Figure 1C). The first two PCs explained 5.8 % and 4.6 % of the total variation. Both PC1 and PC2 separate the samples into three clusters. Sampling locations do not form individual groups within the three main clusters, reflecting the high amount of admixture observed in Figure 1B, except for Madeira and Azores, which appear separated in the PC1.
The main clusters observed in the mitochondrial haplotype network combine haplotypes that are not geographically confined to a region, suggestive of gene flow mainly between Center and East (Figure 1D), which is in agreement with their proximity in PC2 (Figure 1C). The West group dominates a centrally branching haplogroup.