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Ancestry composition of sites for each port. As in Figure 2, barplots represent the ancestry estimation for individuals at the indicated locations and are ordered from left to right by their Med. M. galloprovincialis ancestry. Barplots at the map edges correspond to distant populations with the least cost path distance from the port indicated in parentheses. The inner-most populations used to fit geographic clines are indicated by the reversed triangles. (a) Le Havre; note that the two distinct main basins (North and South-Port 2000) found in this port were separated for geographic cline analyses; the arrow indicates a site located on the estuary side of the dyke, characterised by a majority of M. edulis individuals. (b) Cherbourg; dates indicate collection year; all other samples were collected in 2017. (c) Saint-Malo. (d) Saint-Nazaire. (e1) Bay of Brest. (e2) Detailed map of the port of Brest and the Élorn estuary, which corresponds to the inset rectangle in panel (e1).
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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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... used the hw.test function of the R package pegas (Paradis, 2010) with 10 4 Monte Carlo permutations and a Benjamini-Yekutieli false discovery rate correction. Markers 604 and 190 were identified as significantly departing from HWE in at least one reference group ( Figure S3). ...
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... port of Le Havre was divided into two independent transects: North and South corresponding to the historic basins and the 'Port 2000' recent installations respectively. The least cost distance from the most inward site in each port (indicated by a triangle in Figure 3) was taken as a proxy for geographic distance and to project geographic relationships on a single axis. For the Bay of Brest, the starting site was taken as the right-most population in Figure 1g, up the Élorn estuary. ...
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... the Bay of Brest, the starting site was taken as the right-most population in Figure 1g, up the Élorn estuary. The three points in the bottom-right corner of Figure 3e containing Med. M. galloprovincialis ancestry were excluded from the fit, to account for discrepancies between least cost path distances and the presence of the dock mussels. Pure M. edulis individuals were removed for the analysis in the bay of Brest and Atl. ...
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... the port of Cherbourg, we were able to analyse several temporal samples between 2003 and 2017 ( Figure 3b). These exhibit a small differentiation between the 2003 sample and later years (2015 and 2016; F ST = 0.0066 and 0.0097, Table S8) and this seems to be driven by a small increase in Med. ...
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... individual ancestries were plotted spatially to assess their distribution in and around the five studied French ports (Figures 3). ...
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... edulis while in the Bay of Brest, the native mussels are Atl. M. galloprovincialis (Figure 3). Saint-Malo and Saint-Nazaire lie on the limits of hybrid zones between M. edulis and M. galloprovincialis. ...
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... surroundings of Saint-Malo are mostly inhabited by Atl. M. 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). ...
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... of the five studied ports (all except Brest) have locked basins where the dock mussels were found. Importantly, dock mussels are nearly all localised inside port infrastructures, and we observed a sharp shift at the port entrance (Figure 3). For the ports of Saint-Nazaire, SaintMalo, Cherbourg and Le Havre only four individuals with Med. ...
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... the ports of Saint-Nazaire, SaintMalo, Cherbourg and Le Havre only four individuals with Med. M. galloprovincialis ancestry were detected in coastal wild populations (out of 341 individuals presented in Figure 3). Those individuals were observed at distances between a few hundred meters to 30 km from the entrance of the ports. ...
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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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... Figure 4: Geographic clines computed with the package hzar in each study ports (except St-Malo, see text). The x-axis is the distance from the most inward point (reversed triangles in figure 3) determined by a least-cost path analysis. Top crosses indicate the distance of each site considered. ...
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... have narrow widths across all ports. Average widths are 3.99 km (SD = 1.80) and (Figure 3a). The interpretation in the Bay of Brest is more difficult due to two factors. ...
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... interpretation in the Bay of Brest is more difficult due to two factors. First, the spread of dock mussels and sympatry with local ones in several populations make allele frequencies more variable between close populations (Figure 3e-f). Second, we had a reduced number of differentiated markers between Atl. and Med. ...
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... example, the mitochondrial marker (601) is differentiated between the Med. and the Atl. M. galloprovincialis lineages ( Figure S38). This locus exhibits large distortions (D) towards the Med. ...
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... examined all pairwise comparisons involving the same parental backgrounds in similar conditions (Figure 5a-[i]): the five dock mussels populations from French ports ('Dock / Dock'), the two Norwegian introductions ('Norway / Norway'), and the natural hybrid zones ('Natural / Natural'). In each case, the allele frequency deviations are significantly and positively correlated between events, with large to medium effect sizes ( Figures 5 and S33-S34). The same was also true when we compared the Norwegian introductions to the natural hybrid zones involving the same M. galloprovincialis genetic background ('Norway / Natural', Figure 5a-[ii]). ...
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... distribution of ancestry track lengths along the genome of admixed individuals. Interestingly, in 1978, Prof. David Skibinski analysed hybrids from natural populations in the Swansea region (UK) with allozymes (Skibinski, Beardmore, & Ahmad, 1978) and noticed that the 'King's dock' populations (Swansea port) were unusual ( Figure S39). Those populations showed linkage and Hardy-Weinberg equilibria, and intermediate allele frequencies between M. edulis and M. galloprovincialis. ...
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Citations
... Bivalve molluscs are a useful model system for investigating the relationship between genomic architecture and adaptation, as there is ample evidence of local adaptation across heterogeneous environments [21][22][23], as well as a growing body of evidence documenting an exceptional degree of genomic structural variation [24][25][26]. King scallop (Pecten maximus), also known as great scallop, is a high-value mollusc that supports a large fishery in the eastern North Atlantic ocean, and for which attempts to describe genetic population structure span decades [27][28][29]. ...
The genomic landscape of divergence—the distribution of differences among populations or species across the genome—is increasingly characterized to understand the role that microevolutionary forces such as natural selection and recombination play in causing and maintaining genetic divergence. This line of inquiry has also revealed chromosome structure variation to be an important factor shaping the landscape of adaptive genetic variation. Owing to a high prevalence of chromosome structure variation and the strong pressure for local adaptation necessitated by their sessile nature, bivalve molluscs are an ideal taxon for exploring the relationship between chromosome structure variation and local adaptation. Here, we report a population genomic survey of king scallop ( Pecten maximus ) across its natural range in the northeastern Atlantic Ocean, using a recent chromosome-level genome assembly. We report the presence of at least three large (12–22 Mb), putative chromosomal inversions associated with sea surface temperature and whose frequencies are in contrast to neutral population structure. These results highlight a potentially large role for recombination-suppressing chromosomal inversions in local adaptation and suggest a hypothesis to explain the maintenance of differences in reproductive timing found at relatively small spatial scales across king scallop populations.
... For species identification, a dataset of 81 common markers was obtained (for more details, see Simon et al, 2019), which allowed identification of all mussels as the species M. edulis. ...
... Our experimental design, using different mussel stocks corresponding to different geographic sources in the different experimental sites, did not allow evaluating the influence of the genetic background on the mussel mortality, that is to say if mussel stocks from different geographic sources would show different susceptibility to mortality outbreaks. Mussels on the French Atlantic coast have complex genetic structure (Bierne et al., 2003;Fly et al., 2015;Michalek et al., 2016, Simon et al, 2019. ...
In 2014, a high and unusual mass mortality of mussels occurred in several important production areas along the French coasts of the Atlantic and English Channel. In the first quarter of 2016, mass mortalities hit farms on the west coast of the country once again. These heterogeneous mortality events elicited a multi-parametric study conducted during the 2017 mussel season in three sites in northern Brittany (Brest, Lannion and St. Brieuc). The objective was to assess the health status of these mussels, follow mortality and attempt to identify potential causes of the abnormal high mortality of farmed mussels in northern Brittany. Brest was the most affected site with 70% cumulative mortality, then Lannion with 40% and finally St. Brieuc with a normal value of 15%. We highlighted a temporal 'mortality window' that opened throughout the spring season, and concerned the sites affected by mortality of harmful parasites (including pathogenic bacteria), neoplasia, metal contamination, and tissue alterations. Likely, the combination of all these factors leads to a weakening of mussels that can cause death.
