title: Genomic signatures of local directional selection in a high gene flow marine organism; the Atlantic cod (Gadus morhua).
abstract: BACKGROUND
  Marine fishes have been shown to display low levels of genetic structuring and associated high levels of gene flow, suggesting shallow evolutionary trajectories and, possibly, limited or lacking adaptive divergence among local populations. We investigated variation in 98 gene-associated single nucleotide polymorphisms (SNPs) for evidence of selection in local populations of Atlantic cod (Gadus morhua L.) across the species distribution.
  
  
  RESULTS
  Our global genome scan analysis identified eight outlier gene loci with very high statistical support, likely to be subject to directional selection in local demes, or closely linked to loci under selection. Likewise, on a regional south/north transect of central and eastern Atlantic populations, seven loci displayed strongly elevated levels of genetic differentiation. Selection patterns among populations appeared to be relatively widespread and complex, i.e. outlier loci were generally not only associated with one of a few divergent local populations. Even on a limited geographical scale between the proximate North Sea and Baltic Sea populations four loci displayed evidence of adaptive evolution. Temporal genome scan analysis applied to DNA from archived otoliths from a Faeroese population demonstrated stability of the intra-population variation over 24 years. An exploratory landscape genetic analysis was used to elucidate potential effects of the most likely environmental factors responsible for the signatures of local adaptation. We found that genetic variation at several of the outlier loci was better correlated with temperature and/or salinity conditions at spawning grounds at spawning time than with geographic distance per se.
  
  
  CONCLUSIONS
  These findings illustrate that adaptive population divergence may indeed be prevalent despite seemingly high levels of gene flow, as found in most marine fishes. Thus, results have important implications for our understanding of the interplay of evolutionary forces in general, and for the conservation of marine biodiversity under rapidly increasing evolutionary pressure from climate and fisheries induced changes in local environments.
authors: National Institute of Aquatic Resources, Technical University of Denmark, Vejlsøvej 39, DK-8600 Silkeborg, Denmark. een@aqua.dtu.dk
authors: Einar E
authors: EE
authors: Nielsen
authors: Jakob
authors: J
authors: Hemmer-Hansen
authors: Nina A
authors: NA
authors: Poulsen
authors: Volker
authors: V
authors: Loeschcke
authors: Thomas
authors: T
authors: Moen
authors: Torild
authors: T
authors: Johansen
authors: Christian
authors: C
authors: Mittelholzer
authors: Geir-Lasse
authors: GL
authors: Taranger
authors: Rob
authors: R
authors: Ogden
authors: Gary R
authors: GR
authors: Carvalho
mesh_terms: D000222:Adaptation, Physiological / Q000235:genetics; D000818:Animals; D001267:Atlantic Ocean; D001499:Bayes Theorem; D019143:Evolution, Molecular; D048389:Gadus morhua / Q000235:genetics*; D051456:Gene Flow; D005828:Genetics, Population; D016678:Genome; D009659:North Sea; D020641:Polymorphism, Single Nucleotide; D017422:Sequence Analysis, DNA