Abstract
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 data set 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.
Highlights
Assessment of genetic connectivity among populations is widely used in conservation biology and species management to evaluate effective dispersal and the consequences of spatially-dependent evolutionary processes on species persistence, resilience and adaptability (Cayuela et al 2018; Gagnaire 2020)
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
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. (250 words)
Summary
Assessment of genetic connectivity among populations is widely used in conservation biology and species management to evaluate effective dispersal and the consequences of spatially-dependent evolutionary processes on species persistence, resilience and adaptability (Cayuela et al 2018; Gagnaire 2020). Contemporary ATL genomes are made of ~5% of MED genetic material, while western and eastern MED genomes contain ~31% and 13% of ATL ancestry, respectively (Duranton et al, 2018) Such levels of introgression are sufficiently high to provide information on genetic connectivity within both sea bass lineages. In the Mediterranean, Duranton et al (2019) analyzed the neutral decay of introgressed haplotype length as a function of distance from the contact zone to estimate a mean per-generation dispersal distance of 5 to 50 km This quantitative approach, based on a small number of whole-genome sequences, did not have the spatial resolution required to provide a detailed map of connectivity. We investigate fine-scale connectivity patterns among Atlantic sea bass populations using the spatial diffusion of Mediterranean alleles from southern Portugal toward the northern part of the species range. We find molecular signatures of a recent northward expansion, consistent with recently expanding sea bass fisheries in the northern part of the species range
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