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Location and ancestry composition of sites for reference and admixed populations. Barplots represent ancestries of individuals from the focal site, estimated by Structure with K = 4. In all barplots, individuals have been sorted from left to right by their level of Mediterranean M. galloprovincialis ancestry. Coloured coastlines indicate the approximate distribution of parental genetic background, with colour code as used in Figure 1. Hybrid zones are coloured in purple. Points (a)-(e) correspond to the ports of Le Havre, Cherbourg, Saint-Malo, Saint-Nazaire and Brest respectively, which are detailed in fig. 3.
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Human-mediated transport creates secondary contacts between genetically differentiated lineages, bringing new opportunities for gene exchange. When similar introductions occur in different places, they provide informally replicated experiments for studying hybridisation. We here examined 4279 Mytilus mussels, sampled in Europe and genotyped with 77...
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... followed the kit protocol with modified volumes for the following reagents: 2× diluted magnetic beads, 200 µL of MB3 and MB4, 300 µL of MB5 and 100 µL of MB6. The extraction program is presented in Figure S2. ...
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... then defined groups of individuals used as reference in downstream analyses and identified loci Table 1: Groups used in the analyses of ancestry comparisons and correlations of distortion. The location and ancestry composition of sub-groups are indicated in Figure 2. The native genetic backgrounds possibly encountered is indicated for cases of introduction (n/a: not applicable deviating from Hardy-Weinberg expectations, to filter used markers for analyses depending on equilibrium hypotheses. ...
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... obtain a finer classification in port areas, mussels were assigned to M. edulis, Atl. M. galloprovincialis or dock mussel clusters using the local Structure analysis without admixture (K = 3, Figure S20). See Table S3 for details on the selection thresholds for each group and Figure S21 for independent plots of selected individuals. ...
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... software Newhybrids (Anderson & Thompson, 2002) was used to evaluate the probability that individuals were first or second generation hybrids between the dock mussels and native lineages (Figures S26-S27). ...
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... Structure ancestry estimates with admixture, identifying the four clusters edu_eu_south, gallo_atl, gallo_med and edu_am, were used (K = 4, Figure S21). This selection allowed a homogeneous comparison of ancestry levels between all admixed populations ( Figure S23). ...
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... galloprovincialis groups from Brittany were identified and their ancestries were compared: (i) mussels distant from the Bay of Brest, Northern Brittany population (gallo_atl_brit); (ii) individuals outside the Bay of Brest (the limit being the entrance straight), taken as reference local individuals; and (iii) individuals inside the Bay of Brest classified as Atl. M. galloprovincialis with the local Structure without admixture result ( Figure S20). ...
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... edulis and are called 'naturally admixed' (Figure 1c, Table 1). This category includes geographically distant samples from Scotland (ABD), the English Channel island of Jersey (JER), the Murchison oil platform in the Norwegian Sea (MCH) and the natural hybrid zone in South Brittany (HZSB, Figure 2). As far as we know, these groups are free from human-mediated introductions. ...
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... admixed populations cover much of the range of admixture proportions observed between the two parental species ( Figure S23). These four populations exhibit significant differences in their Atl. ...
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... edulis ancestry, contrasting with the Atl. M. galloprovincialis ancestry excess of the three other natural populations (Figures 2 and S23). Atl. ...
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... https://doi.org/10.1101/590737 doi: bioRxiv preprint S23). The presence of individuals with some Atl. ...
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... French ports, and more rarely in their vicinity. They exhibit a characteristic admixture between Med. M. galloprovincialis and South-Eu. M. edulis, and are defined as the intermediate cluster between these two lineages ( Figure 1, Table. 1). The selection of individuals defined as dock mussels is based on a Structure analysis without admixture ( Figure. S20). Dock mussels are closer to Med. M. galloprovincialis than to M. edulis in the PCA, reflecting the estimated ancestries, and are not differentiated by other axes of the PCA (Figure 1a). Additionally, they show a large variance in all directions, presumably including inter-specific hybrids with M. edulis and inter-lineage hybrids with ...
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... implies that no natural hybridisation is observed between these two lineages in our dataset. This is in accordance with the distribution of the Mytilus lineages (Figure 2). ...
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... frequencies of dock mussels for markers differentiated between M. edulis and Med. M. galloprovincialis are also consistent with the observed levels Figure S22). All port populations are highly similar, both spatially and temporally, in their variance of allele frequencies regardless of their overall level of introgression ( Figure S22). ...
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... galloprovincialis are also consistent with the observed levels Figure S22). All port populations are highly similar, both spatially and temporally, in their variance of allele frequencies regardless of their overall level of introgression ( Figure S22). ...
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... comparing ports, Cherbourg, Saint-Nazaire and Saint-Malo are the least introgressed populations ( Figure S23, Table S11). Le Havre appear to be the most introgressed by SouthEu. ...
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... estimate the age of the admixture event which resulted in the dock mussels, we inferred levels of linkage disequilibria ( Figure S24). Disequilibria were present, but at low levels indicating that there had been several generations of recombination since admixture. ...
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... ports of interest are localised in regions characterised by different native species (Fig-ure 2). The native species around Le Havre and Cherbourg is South-Eu. ...
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... galloprovincialis (Figure 3c), and Saint-Nazaire is located in a zone mostly composed of M. edulis with the presence of Atl. M. galloprovincialis in sympatry (Figure 2 and 3d). Around the latter, local M. galloprovincialis are more introgressed by M. edulis than those found in Brittany as they lie at the far end of the South Brittany hybrid zone ( Bierne et al., 2003). ...
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... the opposite direction (from the natural coast to the port), we mainly find native migrants close to the port entrance inside Le Havre, Cherbourg and Saint-Nazaire (Figure 3). Le Havre and Saint-Nazaire are the ports containing the largest number of M. edulis migrants, yet Le Havre is the only one where F1 hybrids between dock mussels and M. edulis have been observed (identified with Newhybrids, Figure S26). ...
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... we note that the tail of the distribution of Med. M. galloprovincialis ancestry in the Bay of Brest is skewed towards higher values ( Figure S23). This tail is due to the presence of hybrids between dock mussels and the local native Atl. ...
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... tail is due to the presence of hybrids between dock mussels and the local native Atl. M. galloprovincialis ( Figure S27). ...
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... correlations were strongest for lab backcrosses (BCs), and much weaker and non-significant for the F2. This is consistent with the genetic makeup of the dock mussels, which have hybrid indexes closer to BC genotypes than to F2s ( fig S23 and S25), albeit more recombined. Globally, the level and consistency of correlations increases with the similarity between admixture events (from groups [iv] to [i] in Figure 5). ...
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... galloprovincialis ancestry, which may sometimes be called a 'hybrid swarm' due to a uni-modal distribution of hybrid indices and a complete mixing of ancestries along the genome (Allendorf, Leary, Spruell, & Wenburg, 2001;Beninde, Feldmeier, Veith, & Hochkirch, 2018;Jiggins & Mallet, 2000). We additionally show that there is ongoing secondary admixture between the dock mussel cluster and native genetic backgrounds, exemplified by the detection of F1 hybrids in Le Havre ( Figure S26). While no F1 hybrids have been identified in the Bay of Brest by Newhybrids ( Figure S27) -which most probably results from reduced power of identification between the two M. galloprovincialis lineages -the distribution of ancestries observed leaves little doubt that hybridisation is ongoing between dock mussels and Atl. ...
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... additionally show that there is ongoing secondary admixture between the dock mussel cluster and native genetic backgrounds, exemplified by the detection of F1 hybrids in Le Havre ( Figure S26). While no F1 hybrids have been identified in the Bay of Brest by Newhybrids ( Figure S27) -which most probably results from reduced power of identification between the two M. galloprovincialis lineages -the distribution of ancestries observed leaves little doubt that hybridisation is ongoing between dock mussels and Atl. M. galloprovincialis (Figures 3 and S23). ...
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... no F1 hybrids have been identified in the Bay of Brest by Newhybrids ( Figure S27) -which most probably results from reduced power of identification between the two M. galloprovincialis lineages -the distribution of ancestries observed leaves little doubt that hybridisation is ongoing between dock mussels and Atl. M. galloprovincialis (Figures 3 and S23). Given the possibilities of local admixture, the relative global homogeneity of dock mussels could be explained either by the recentness of the introduction, by the existence of extrinsic or intrinsic barriers to introgressions, or by both. ...
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... galloprovincialis in all sampled ports provide arguments for this hypothesis. For instance in the Bay of Brest or in Saint-Malo, the presence of dock mussels with similar genetic compositions to the other ports (Figure S23), where the local native species is however different (i.e., predominantly Atl. M. galloprovincialis rather than M. edulis), suggests that the admixture with M. edulis happened before the introduction of dock mussels in these ports. ...
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... our sampling around ports was not exhaustive, dock mussels do appear to be restricted to the port interiors, with only a few introduced mussels detected in distant populations. While the presence of introduced migrants up to 30 km from ports may appear concerning, most distant individuals are hybrids between dock mussels and the local background (Figures 3 and S26-S27). Therefore, we can hypothesise that the propagule pressure from ports will be swamped by large native populations for most of the ports. ...
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