Abstract
Previous genetic studies of Atlantic swordfish (Xiphias gladius L.) revealed significant differentiation among Mediterranean, North Atlantic and South Atlantic populations using both mitochondrial and nuclear DNA data. However, limitations in geographic sampling coverage, and the use of single loci, precluded an accurate placement of boundaries and of estimates of admixture. In this study, we present multilocus analyses of 26 single nucleotide polymorphisms (SNPs) within 10 nuclear genes to estimate population differentiation and admixture based on the characterization of 774 individuals representing North Atlantic, South Atlantic, and Mediterranean swordfish populations. Pairwise F ST values, AMOVA, PCoA, and Bayesian individual assignments support the differentiation of swordfish inhabiting these three basins, but not the current placement of the boundaries that separate them. Specifically, the range of the South Atlantic population extends beyond 5°N management boundary to 20°N-25°N from 45°W. Likewise the Mediterranean population extends beyond the current management boundary at the Strait of Gibraltar to approximately 10°W. Further, admixture zones, characterized by asymmetric contributions of adjacent populations within samples, are confined to the Northeast Atlantic. While South Atlantic and Mediterranean migrants were identified within these Northeast Atlantic admixture zones no North Atlantic migrants were identified respectively in these two neighboring basins. Owing to both, the characterization of larger number of loci and a more ample spatial sampling coverage, it was possible to provide a finer resolution of the boundaries separating Atlantic swordfish populations than previous studies. Finally, the patterns of population structure and admixture are discussed in the light of the reproductive biology, the known patterns of dispersal, and oceanographic features that may act as barriers to gene flow to Atlantic swordfish.
Highlights
The epipelagic realm of the world’s oceans is a relatively homogeneous and contiguous environment characterized by the near absence of physical barriers
The power assignment of each loci using WHICHLOCI v1.0 identified CaM as the locus with the highest power of discrimination for North and South Atlantic populations followed by Mlc2, signal recognition particle 54 (SRP54), AldB, acidic ribosomal phosphoprotein P0 (ARP), Act2α, lactose dehydrogenase A (ldhA), GpHR, ATPsβ, and adenine nucleotide translocator (ANT) respectively
CaM displayed the lowest power of discrimination between North Atlantic and Mediterranean populations, with Mlc2 yielding the highest power of discrimination between them followed by ATPsβ, Act2α, ldhA, ANT, AldB, SRP54, ARP, and GpHR, respectively
Summary
The epipelagic realm of the world’s oceans is a relatively homogeneous and contiguous environment characterized by the near absence of physical barriers. These features when coupled with the high dispersal behavior and demographic characteristics of many marine species result in high levels of gene flow [1]. Significant inter-oceanic differentiation, has been documented in many highly migratory fishes, including bigeye tuna [7], Atlantic bluefin tuna [8, 9], albacore tuna [10, 11], and many species of billfishes [12] and swordfish [13]. Swordfish (Xiphias gladius L.) shows significant genetic differentiation within the Atlantic Ocean [13, 17, 18] and within the Mediterranean Sea [19]
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