Biology subjects

Bierne, N.

Publications and source records attributed to Bierne, N..

At least 19 recordsLinked to original sources

bioRxiv2026

Adaptive introgression in subdivided populations leaves distinct genomic footprints under global and local selection: a case study in Ciona intestinalis

Adaptive introgression, the transfer of beneficial alleles between species through hybridization, is increasingly recognized as an important source of adaptation to novel environments. However, the theory of adaptive introgression has primarily been developed for a single panmictic population, whereas many empirical examples involve spatially subdivided populations. In such systems, a low-frequency adaptive allele may either represent the leading edge of an adaptive travelling wave or occur in a subpopulation where the introgressed allele is locally selected against. We developed a simple prediction based on the relationship between adaptive allele frequency and introgressed tract length. Simulations predict a positive correlation during an ongoing sweep of an unconditionally beneficial allele, but no correlation under local adaptation. Tract erosion provides a recombination clock for time since introgression, which also determines the distance travelled by the adaptive wave and hence allele frequency in the former, whereas allele frequency is determined by local selection in the latter. We tested this prediction in a well-characterized case of adaptive introgression, in the sea squirt Ciona intestinalis, using haplotagging to reconstruct phased genomes and local ancestry. We found no relationship between adaptive allele frequency and introgressed tract length, providing additional support for a model of local adaptation rather than ongoing global selective sweep.

genetics↗

bioRxiv2026

Parallel differentiation extends beyond chromosomal inversions to small genomic islands on a background of rampant gene flow in the European flat oyster (Ostrea edulis)

Heterogeneous genomic landscapes of differentiation can reveal evolutionary processes acting over different timescales. Whereas genetic structure in the European flat oyster, Ostrea edulis, is mainly shaped by geography, previous work suggested parallel differentiation between western and eastern Mediterranean populations and between Atlantic and North Sea populations, resulting in genetic convergence between range margin populations in some genomic regions. Here, we analyzed 27 oysters from four locations using moderate-coverage (~20X) short-read whole-genome sequencing. We first examined three known chromosomal inversions and found evidence that they predate divergence with the sister species O. chilensis. We then scanned the genome for parallel differentiation at the individual-SNP rather than sliding-window scale using the product of allele-frequency differences across the two geographic contrasts. Two of the three inversions were detected, but we also identified 12 small non-inverted genomic regions showing even stronger signals of parallel differentiation. These 'islands of parallelism' typically overlap one or a few genes, and most showed elevated dXY consistent with long-term barriers to gene flow. Two islands overlap nuclear ATP synthase subunits, while mitochondrial ATP8 shows a convergent amino-acid change, suggesting possible nuclear-nuclear and mitonuclear incompatibilities. Finally, demographic inference with heterogeneous gene flow supported post-glacial secondary contact and a lineage-based rather than geography-based population topology. Our results suggest that the genomes of O. edulis lineages have been largely porous to gene flow upon secondary contact, except for chromosomal inversions and small genomic islands. These regions likely act as barrier loci, preserving a signal of a shared evolutionary history among marginal populations that has otherwise been erased by rampant gene flow.

evolutionary biology↗

bioRxiv2026

A 23-copy tandem repeat of a cytochrome P450 gene underlies an adaptive introgression breakthrough in Ciona intestinalis

Introgression between highly divergent species is generally selected against, yet some foreign alleles can break through species barriers when they provide a sufficient selective advantage, a process known as adaptive introgression. Here, we characterize the structural variant underlying adaptive introgression from Ciona robusta into C. intestinalis, two species separated by 14.5% net molecular divergence. Previous work identified a narrow peak of C. robusta ancestry on chromosome 5 that reached high frequency in several C. intestinalis populations despite otherwise restricted introgression. Using whole-genome Nanopore sequences from 15 C. intestinalis and three C. robusta individuals, together with short-read datasets including parent-offspring trios, we narrowed down the core introgressed segment from 16 to three genes and characterized copy-number variation affecting the cytochrome P450 gene CYP2U1c. The introgressed haplotype carries a tandem amplification of a 5,173-bp segment containing CYP2U1c, with 23 consecutive copies and conserved repeat boundaries. The nearly 120-kb array could not be fully resolved by long-read sequencing, but coverage consistently supported approximately 23 copies per introgressed haplotype. Despite its extreme size and repetitive structure, the allele showed Mendelian inheritance. CYP2U1c copy number was also polymorphic in C. robusta. Long reads fully resolved two structurally distinct haplotypes in one individual, carrying either a duplication encompassing adjacent CYP2U1b and CYP2U1c genes or five CYP2U1c copies. Their distinct breakpoints indicate independent origins. Our results reveal independently arising CYP2U1c amplifications in the donor species and show that an extreme high-copy-number allele was subsequently favoured following introgression into C. intestinalis.

evolutionary biology↗

bioRxiv2026

A cryptic local genetic cluster in Northern France amid the European mosaic of flat oyster lineages revealed by integrating SNP array and whole-genome sequencing

The European flat oyster (Ostrea edulis), like numerous other oyster species, has been exploited for millennia and cultivated and translocated for centuries. Following a severe population decline, and in the context of ongoing conservation and restoration programs, genetic considerations must now be addressed to avoid mistakes. The objective of our study was to complement population genetic studies conducted at various scales along European coasts. Our sampling primarily targeted the French Atlantic, English Channel, and Mediterranean coasts, aiming to provide a fine-scale genetic characterization of populations in these regions. By integrating SNP array and low-coverage sequencing datasets, we obtained a comprehensive overview of the population genetic structure of Ostrea edulis across western Europe. Most previously identified clusters in Western Europe were confirmed. In France, populations assigned to these clusters exhibited notable within-patch homogeneity. However, two key findings emerged: (1) an extensive overlap zone between the Atlantic and western Mediterranean clusters, spanning at least from southern Portugal to southern France, and (2) the detection of a novel, clearly distinct cryptic cluster east of the English Channel, whose geographic range remains to be better delineated. These insights are critical for informing management decisions, particularly as restoration and conservation plans are currently being implemented across the species range.

genetics↗

bioRxiv2026

Localised admixture triggers parallel adaptation in a coral species trio on the Great Barrier Reef

Hybridisation can shape evolutionary trajectories and fuel rapid adaptation. Yet, whether adaptive introgression repeatedly reconfigures genomes in predictable ways remains largely unknown. Here, we show that three-way admixture drives repeatable parallel differentiation in reef-building corals in the Acropora hyacinthus species complex. Adaptive introgression is confined to the southern Great Barrier Reef, where admixture timing coincides with a recent breakdown in species barriers despite limited historical gene flow. Genome-wide introgression is characterised by numerous shared minor-parent ancestry peaks that are enriched for stress-response genes and under positive selection for parallel adaptive admixture. Our results demonstrate that introgressive hybridisation can be geographically restricted between sympatric marine species and can promote repeated rapid adaptation that transcends species boundaries.

evolutionary biology↗

bioRxiv2025

k-mer spectra and allelic coverage analyses reveal pervasive polymorphic duplications in Ostrea edulis

Oysters are known for their high genetic diversity, potentially high genetic load, and structurally complex genomes. The European flat oyster, Ostrea edulis, is no exception. Here, we combined a reference-free analysis of pairs of k-mers differing by a single nucleotide (hereafter: het-mers) with a reference-based analysis of allelic coverage using high-coverage short-read sequencing data (70-160X) from five individuals. Up to one-third of SNPs called as heterozygous within an individual exhibited allelic coverage fractions departing from the expected 0.5 ratio. Despite no evidence of recent whole-genome duplication, 40% of het-mers displayed coverage profiles consistent with duplicated genomic segments. We classified AAB het-mers as CN3 and AAAB or AABB het-mers as CN4, corresponding to nucleotide variants located within segments present in three and four copies, respectively, and interpreted as heterozygous and homozygous duplications. These CN3 and CN4 het-mers were typically observed in only a subset of individuals and mostly switched among CN3, CN4, or absence from the het-mer catalog, providing evidence for pervasive paralogous variants associated with rampant polymorphic duplications of genomic segments that vary in copy number among individuals. Reference-based analysis of sequencing depth and allelic coverage fraction similarly showed that SNPs with deviant allelic coverage are frequently associated with duplicated genomic segments segregating within populations, predominantly at low frequencies. Despite their different principles and potential biases, both approaches converged on a minimum estimate of 15% of nucleotide variants being associated with polymorphic duplications. Duplication-associated SNPs occurred in coding and non-coding regions to a similar extent, but their distribution was heterogeneous across chromosomes, with the highest densities on three of the ten chromosomes. These three chromosomes are non-metacentric and particularly prone to chromosome loss in oysters, which are known for their high incidence of somatic aneuploidy. This pattern supports two non-exclusive hypotheses: an architecture-driven model in which chromosome properties prone to structural instability promote both duplications and somatic aneuploidy, and a compensation-driven model in which duplications evolve to buffer dosage imbalance in aneuploid cells. Finally, we discuss how extensive polymorphic duplication may bias population genomic analyses in O. edulis and other ostreids.

genomics↗

bioRxiv2024

Comparative population genomics unveils congruent secondary suture zone in Southwest Pacific Hydrothermal Vents

How the interplay of biotic and abiotic factors shapes current genetic diversity at the community level remains an open question, particularly in the deep sea. Comparative phylogeography of multiple species can reveal the influence of past climatic events, geographic barriers, and species life history traits on spatial patterns of genetic structure across lineages. To shed light on the factors that shape community-level genetic variation and to improve our understanding of deep-sea biogeographic patterns, we conducted a comparative population genomics study on seven hydrothermal vent species co-distributed in the Back-Arc Basins (BABs) of the Southwest Pacific region. Using ddRAD-seq, we compared the range-wide distribution of genomic diversity across species and discovered a shared phylogeographic break. Demogenetic inference revealed shared histories of lineage divergence and a secondary contact. Low levels of asymmetric gene flow probably occurred in most species between the Woodlark and North Fiji basins, but the exact location of contact zones varied from species to species. For two species, we found individuals from the two lineages co-occurring in sympatry in Woodlark Basin. Although species exhibit congruent patterns of spatial structure (Eastern vs Western sites), they also show variation in the degree of divergence among lineages across the suture zone. Our results also show heterogeneous gene flow across the genome, indicating possible partial reproductive isolation between lineages and early speciation. Our comparative study highlights the pivotal role of historical and contemporary factors, underscoring the need for a comprehensive approach--especially in addressing knowledge gaps on the life history traits of deep-sea species.

evolutionary biology↗

bioRxiv2023

Falling shoulders ahead: adaptive alien genes are maintained amid vanishing introgression footprint in sea squirts

Human transport of species across oceans disrupts natural dispersal barriers and facilitates hybridisation between previously allopatric species. The recent introduction of the North Pacific sea squirt, Ciona robusta, into the native range of the North Atlantic sea squirt, C. intestinalis, is a good example of this outcome. Recent studies have revealed an adaptive introgression, in a single chromosomal region, from the introduced into the native species. Here, we monitored this adaptive introgression over time, examining both the frequency of adaptive alien genes at the core and the hitchhiking footprint in the shoulders of the introgression island, by studying a thousand Ciona spp. individuals collected in 22 ports of the contact zone, 14 of which were sampled 20 generations apart. For that purpose, we developed a KASP multiplex genotyping approach, which proved effective in identifying native, non-indigenous and hybrid individuals and in detecting introgressed haplotypes. Adaptive alien genes were detected where they had been found 20 generations ago, as well as in some newly sampled ports. The core region of the introgression, where the frequency of C. robusta alleles is the highest and locally adaptive genes must be, remains stable in both space and time. In contrast, we observed an erosion of C. robusta ancestry tracts in flanking chromosomal shoulders on the edges of the core, consistent with the second phase of a local sweep and a purge of incompatible introgressed alleles. Our study reveals how the shoulders of an adaptive introgression island fall over time after the initial sweep.

evolutionary biology↗

bioRxiv2023

Divergence and gene flow history at two large chromosomal inversions involved in long-snouted seahorse ecotype formation

Chromosomal inversions can play an important role in divergence and reproductive isolation by building and maintaining distinct allelic combinations between evolutionary lineages. Alternatively, they can take the form of balanced polymorphisms that segregate within populations over time until one arrangement becomes fixed. Many questions remain about how these different inversion polymorphisms arise, how the mechanisms responsible for their long-term maintenance interact, and ultimately how they contribute to speciation. The long-snouted seahorse (Hippocampus guttulatus) is known to be subdivided into partially isolated lineages and marine-lagoon ecotypes differentiated by structural variation. Here, we aim to characterise these differences along the entire genome, and to reconstruct their history and role in ecotype formation. We generated a near chromosome-level reference genome assembly and described genome-wide patterns of diversity and divergence through the analysis of 112 whole-genome sequences from Atlantic, Mediterranean, and Black Sea populations. Combined with linked-read sequencing data, we found evidence for two megabase-scale chromosomal inversions showing contrasted allele frequency patterns across the species range. We reveal that these inversions represent ancient intraspecific polymorphisms, one being likely maintained by divergent selection, and the other by associative overdominance. Haplotype combinations characterising Mediterranean ecotypes also suggest the existence of potential interactions between the two inversions, possibly driven by environment-dependent fitness effects. Lastly, we detected gene flux eroding divergence between inverted alleles at varying levels between the two inversions, with a likely impact on their long-term dynamics.

genomics↗

bioRxiv2023

Marine transmissible cancer navigates urbanised waters, threatening to spillover

Inter-individual transmission of cancer cells represents a unique form of microparasites increasingly reported in marine bivalves. In this study, we sought to understand the ecology of the propagation of Mytilus trossulus Bivalve Transmissible Neoplasia 2 (MtrBTN2), a transmissible cancer affecting four Mytilus mussel species worldwide. We investigated the prevalence of MtrBTN2 in the mosaic hybrid zone of M. edulis and M. galloprovincialis along the French Atlantic coast, sampling contrasting natural and anthropogenic habitats. We observed a similar prevalence in both species, likely due to the spatial proximity of the two species in this region. Our results showed that ports had higher prevalence of MtrBTN2, with a possible hotspot observed at a shuttle landing dock. No cancer was found in natural beds except for two sites close to the hotspot, suggesting spillover. Ports may provide favourable conditions for the transmission of MtrBTN2, such as high mussel density, stressful conditions, sheltered and confined shores, or buffered temperatures. Ships may also spread the disease through biofouling. Our results suggest ports may serve as epidemiological hubs, with maritime routes providing artificial gateways for MtrBTN2 propagation. This highlights the importance of preventing biofouling on docks and ship hulls to limit the spread of marine pathogens hosted by fouling species.

ecology↗

bioRxiv2023

Transcriptomics of mussel transmissible cancer MtrBTN2 reveals accumulation of multiple cancerous traits and oncogenic pathways shared among bilaterians

Transmissible cancer cell lines are rare biological entities giving rise to diseases at the crossroads of cancer and parasitic diseases. These malignant cells have acquired the amazing capacity to spread from host to host. They have been described only in dogs, Tasmanian devils and marine bivalves. The Mytilus trossulus Bivalve Transmissible Neoplasia 2 (MtrBTN2) lineage has even acquired the capacity to spread inter-specifically between marine mussels of the Mytilus edulis complex worldwide. To identify the oncogenic processes underpinning the biology of these atypical cancers we performed transcriptomics of MtrBTN2 cells. Differential expression, enrichment, protein-protein interaction network, and targeted analyses were used. Overall, our results suggest the accumulation of multiple cancerous traits that way be linked to the long-term evolution of MtrBTN2. We also highlight that vertebrate and lophotrochozoan cancers could share a large panel of common drivers, which supports the hypothesis of an ancient origin of oncogenic processes in bilaterians.

cancer biology↗

bioRxiv2022

Introgression between highly divergent sea squirt genomes: an adaptive breakthrough?

AO_SCPLOWBSTRACTC_SCPLOWHuman-mediated introductions are reshuffling species distribution on a global scale. Consequently, an increasing number of allopatric taxa are now brought into contact, promoting introgressive hybridization between incompletely isolated species and new adaptive gene transfer. The broadcast spawning marine species, Ciona robusta, has been recently introduced in the native range of its sister taxa, Ciona intestinalis, in the English Channel and North-East Atlantic. These sea squirts are highly divergent, yet hybridization has been reported by crossing experiments and genetic studies in the wild. Here, we examined the consequences of secondary contact between C. intestinalis and C. robusta in the English Channel. We produced genomes phased by transmission to infer the history of divergence and gene flow, and analyzed introgressed genomic tracts. Demographic inference revealed a history of secondary contact with a low overall rate of introgression. Introgressed tracts were short, segregating at low frequency, and scattered throughout the genome, suggesting traces of past contacts during the last 30 ky. However, we also uncovered a hotspot of introgression on chromosome 5, characterized by several hundred kb-long C. robusta haplotypes segregating in C. intestinalis, that introgressed during contemporary times the last 75 years. Although locally more frequent than the baseline level of introgression, C. robusta alleles are not fixed, even in the core region of the introgression hotspot. Still, linkage-disequilibrium patterns and haplotype-based tests suggest this genomic region is under recent positive selection. We further detected in the hotspot an over-representation of candidate SNPs lying on a cytochrome P450 gene with a high copy number of tandem repeats in the introgressed alleles. Cytochromes P450 are a superfamily of enzymes involved in detoxifying exogenous compounds, constituting a promising avenue for functional studies. These findings support that introgression of an adaptive allele is possible between very divergent genomes and that anthropogenic hybridization can provide the raw material for adaptation of native lineages in the Anthropocene.

evolutionary biology↗

bioRxiv2021

An introgression breakthrough left by an anthropogenic contact between two ascidians

Human-driven translocations of species have diverse evolutionary consequences such as promoting hybridization between previously geographically isolated taxa. This is well-illustrated by the solitary tunicate, Ciona robusta, native to the North East Pacific and introduced in the North East Atlantic. It is now co-occurring with its congener C. intestinalis in the English Channel, and C. roulei in the Mediterranean Sea. Despite their long allopatric divergence, first and second generation crosses showed a high hybridization success between the introduced and native taxa in the laboratory. However, previous genetic studies failed to provide evidence of recent hybridization between C. robusta and C. intestinalis in the wild. Using SNPs obtained from ddRAD-sequencing of 397 individuals from 26 populations, we further explored the genome-wide population structure of the native Ciona taxa. We first confirmed results documented in previous studies, notably i) a chaotic genetic structure at regional scale, and ii) a high genetic similarity between C. roulei and C. intestinalis, which is calling for further taxonomic investigation. More importantly, and unexpectedly, we also observed a genomic hotspot of long introgressed C. robusta tracts into C. intestinalis genomes at several locations of their contact zone. Both the genomic architecture of introgression, restricted to a 1.5 Mb region of chromosome 5, and its absence in allopatric populations suggest introgression is recent and occurred after the introduction of the non-indigenous species. Overall, our study shows that anthropogenic hybridization can be effective in promoting introgression breakthroughs between species at a late stage of the speciation continuum.

evolutionary biology↗

bioRxiv2021

Prevalence and polymorphism of a mussel transmissible cancer in Europe

Transmissible cancers are parasitic malignant cell lineages that acquired the ability to infect new hosts from the same species, or sometimes related species. First described in dogs and Tasmanian devils, transmissible cancers were later discovered in some marine bivalves affected by a leukemia-like disease. In Mytilus mussels, two lineages of Bivalve Transmissible Neoplasia (BTN), both emerged in a M. trossulus founder individual, have been described to date (MtrBTN1 and MtrBTN2). Here, we performed an extensive screening of genetic chimerism, a hallmark of transmissible cancer, by genotyping hundred SNPs of thousands of European Mytilus mussels. The genetic analysis allowed us to simultaneously obtain the genotype of hosts -M. edulis, M. galloprovincialis or hybrids- and the genotype of tumors of heavily infected individuals. In addition, a subset of individuals were systematically genotyped and analysed by histology in order to screen for possible non-transmissible cancers. We detected MtrBTN2 at low prevalence in M. edulis, and also in M. galloprovincialis and hybrids although at a much lower prevalence. No MtrBTN1 or new BTN were found but a few individuals with non-transmissible neoplasia were observed at a single polluted site on the same sampling date. We observed a diversity of MtrBTN2 genotypes that appeared more introgressed or more ancestral than MtrBTN1 and reference healthy M. trossulus individuals. The observed polymorphism is most likely due to somatic null alleles caused by structural variations or point mutations in primer-binding sites leading to enhanced detection of the host alleles. Despite low prevalence, two divergent sublineages, confirmed by mtCOI sequences, are co-spreading in the same geographic area, suggesting a complex diversification of MtrBTN2 since its emergence and host species shift.

evolutionary biology↗

bioRxiv2020

DILS : Demographic Inferences with Linked Selection by using ABC

We present DILS, a deployable statistical analysis platform for conducting demographic inferences with linked selection from population genomic data using an Approximate Bayesian Computation framework. DILS takes as input single-population or two-population datasets (multilocus fasta sequences) and performs three types of analyses in a hierarchical manner, identifying: 1) the best demographic model to study the importance of gene flow and population size change on the genetic patterns of polymorphism and divergence, 2) the best genomic model to determine whether the effective size Ne and migration rate N.m are heterogeneously distributed along the genome (implying linked selection) and 3) loci in genomic regions most associated with barriers to gene flow. Also available via a web interface, an objective of DILS is to facilitate collaborative research in speciation genomics. Here, we show the performance and limitations of DILS by using simulations, and finally apply the method to published data on a divergence continuum composed by 28 pairs of Mytilus mussel populations/species.

evolutionary biology↗

bioRxiv2019

The Geometry and Genetics of Hybridization

We develop an analytical framework for predicting the fitness of hybrid genotypes, based on Fishers geometric model. We first show that all of the model parameters have a simple geometrical and biological interpretation. Hybrid fitness decomposes into intrinsic effects of hybridity and heterozygosity, and extrinsic measures of the (local) adaptedness of the parental lines; and all of these correspond to distances in a phenotypic space. We also show how these quantities change over the course of divergence, with convergence to a characteristic pattern of intrinsic isolation. Using individual-based simulations, we then show that the predictions apply to a wide range of population genetic regimes, and divergence conditions, including allopatry and parapatry, local adaptation and drift. We next connect our results to the quantitative genetics of line crosses in variable or patchy environments. This relates the geometrical distances to quantities that can be estimated from cross data, and provides a simple interpretation of the "composite effects" in the quantitative genetics partition. Finally, we develop extensions to the model, involving selectively-induced disequilibria, and variable phenotypic dominance. The geometry of fitness landscapes provides a unifying framework for understanding speciation, and wider patterns of hybrid fitness.

evolutionary biology↗

bioRxiv2019

Local introgression at two spatial scales in mosaichybrid zones of mussels

The Mytilus complex of marine mussel species forms a mosaic of hybrid zones, found across temperate regions of the globe. This allows us to study "replicated" instances of secondary contact between closely-related species. Previous work on this complex has shown that local introgression is both widespread and highly heterogeneous, and has identified SNPs that are outliers of differentiation between lineages. Here, we developed an ancestry-informative panel of such SNPs. We then compared their frequencies in newly-sampled populations, including samples from within the hybrid zones, and parental populations at different distances from the contact. Results show that close to the hybrid zones, some outlier loci are near to fixation for the heterospecific allele, suggesting enhanced local introgression, or the local sweep of a shared ancestral allele. Conversely, genomic cline analyses, treating local parental populations as the reference, reveal a globally high concordance among loci, albeit with a few signals of asymmetric introgression. Enhanced local introgression at specific loci is consistent with the early transfer of adaptive variants after contact, possibly including asymmetric bistable variants (Dobzhansky-Muller incompatibilities), or haplotypes loaded with fewer deleterious mutations. Having escaped one barrier, however, these variants can be trapped or delayed at the next barrier, confining the introgression locally. These results shed light on the decay of species barriers during phases of contact.

evolutionary biology↗

bioRxiv2019

Replicated anthropogenic hybridisations reveal parallel patterns of admixture in marine mussels.

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 ancestry informative markers. We identified a type of introduced mussels, called dock mussels, associated with port habitats and displaying a particular genetic signal of admixture between M. edulis and the Mediterranean lineage of M. galloprovincialis. These mussels exhibit similarities in their ancestry compositions, regardless of the local native genetic backgrounds and the distance separating colonised ports. We observed fine-scale genetic shifts at the port entrance, at scales below natural dispersal distance. Such sharp clines do not fit with migration-selection tension zone models, and instead suggest habitat choice and early stage adaptation to the port environment, possibly coupled with connectivity barriers. Variations in the spread and admixture patterns of dock mussels seem to be influenced by the local native genetic backgrounds encountered. We next examined departures from the average admixture rate at different loci, and compared human-mediated admixture events, to naturally admixed populations and experimental crosses. When the same M. galloprovincialis background was involved, positive correlations in the departures of loci across locations were found; but when different backgrounds were involved, no or negative correlations were observed. While some observed positive correlations might be best explained by a shared history and saltatory colonisation, others are likely produced by parallel selective events. Altogether, genome-wide effect of admixture seems repeatable, and more dependent on genetic background than environmental context. Our results pave the way towards further genomic analyses of admixture, and monitoring of the spread of dock mussels both at large and fine spacial scales.

evolutionary biology↗