Spatial gradients of introgressed ancestry reveal cryptic connectivity patterns in a high gene flow marine fish

• Published in: Molecular Ecology
• Main Authors: Robinet, Tony, Roussel, Valérie, Cheze, Karine, Gagnaire, Pierre-Alexandre
• Other Authors: Biologie des Organismes et Ecosystèmes Aquatiques (BOREA), Université de Caen Normandie (UNICAEN), Normandie Université (NU)-Normandie Université (NU)-Muséum national d'Histoire naturelle (MNHN)-Institut de Recherche pour le Développement (IRD)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université des Antilles (UA), Station de Biologie Marine de Concarneau, Direction générale déléguée à la Recherche, à l’Expertise, à la Valorisation et à l’Enseignement-Formation (DGD.REVE), Muséum national d'Histoire naturelle (MNHN)-Muséum national d'Histoire naturelle (MNHN), Institut de Géoarchitecture, Université de Bretagne Sud (UBS)-Université de Brest (UBO)-Institut Brestois des Sciences de l'Homme et de la Société (IBSHS), Université de Brest (UBO)-Université de Brest (UBO), Institut des Sciences de l'Evolution de Montpellier (UMR ISEM), Centre de Coopération Internationale en Recherche Agronomique pour le Développement (Cirad)-École Pratique des Hautes Études (EPHE), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Université de Montpellier (UM)-Institut de recherche pour le développement IRD : UR226-Centre National de la Recherche Scientifique (CNRS), FFP: France Filière PêcheMNHN: Muséum National d'Histoire Naturelle
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2020
• Subjects: [SDV.EE]Life Sciences [q-bio]/Ecology; environment; Northeast Atlantic
• Online Access: https://hal.science/hal-02925509; https://hal.science/hal-02925509/document; https://hal.science/hal-02925509/file/Robinet_etal_MEC_text_REVISED_FINAL.pdf; https://doi.org/10.1111/mec.15611

International audience Assessing genetic connectivity among populations in high gene flow species is sometimes insufficient to evaluate demographic connectivity. Genetic differentiation quickly becomes zero as soon as a few dozen migrants are exchanged per generation. This provides little information to determine whether migration can ensure demographic coupling. The resulting difficulties in delineating conservation units for the management of commercially exploited marine fish species are well illustrated in the case of the European sea bass (Dicentrarchus labrax). Previous attempts to assess connectivity patterns in the northeast Atlantic have been hampered by a lack of spatial genetic structure. In contrast, mark-recapture data suggested low migration rates between regional spawning areas. Here, we show how a spatial gradient of introgressed Mediterranean ancestry across the northeast Atlantic reflects cryptic patterns of genetic and demographic connectivity. Using a 1K SNP chip dataset in 827 individuals sampled from Portugal to the North Sea, we found null overall genetic differentiation across the northeast Atlantic. We however detected a subtle latitudinal admixture gradient originating at the edge of the contact zone with the Mediterranean sea bass lineage. Two significant breaks in the ancestry gradient at the tip of Galicia and northern Brittany indicated barriers to effective dispersal between demographically distinct units. Moreover, a northward expansion signal in Irish and North Seas was revealed by the surfing of rare Mediterranean alleles at the edge of the species range. Our results show that introgressed ancestry gradients offer a powerful alternative to assess genetic and demographic connectivity when the neutral migration-drift balance is not informative.