Allelic variation at nine microsatellite loci and the Pan I locus provides evidence that Atlantic cod (Gadus morhua) around Iceland is genetically structured (F ST = 0.003 and F ST = 0.261, respectively). A total of 2534 cod were sampled at 22 spawning locations. For both types of markers, most of the significant pairwise F ST resulted from northeastern-southwestern comparisons. A multidimensional scaling analysis based on F ST , a spatial hierarchical analysis of molecular variance (SAMOVA) and a hierarchical analysis of molecular variance (AMOVA), conducted on both types of markers confirmed a genetic differentiation between cod from the northeastern and southwestern regions. Genetic discontinuities were revealed across two main current fronts in the southeast and northwest, where the cold and warm water masses meet. The AMOVA also detected genetic differences with depth. Expected and observed heterozygosity of microsatellite loci significantly decreased with depth, whereas the B allele frequency at the Pan I locus increased. A tagging experiment of spawning fish conducted during the same years as the genetic work revealed that tagged individuals released in the southwestern region seldom migrated to the northeastern region and vice versa, suggesting that the southwestern and northeastern populations of Atlantic cod around Iceland represent two distinct spawning components.Résumé : La variation allélique de neuf microsatellites et du locus Pan I met en évidence une structure génétique chez la morue Atlantique (Gadus morhua) dans les eaux islandaises (F ST respectifs, F ST = 0,003 et F ST = 0,261). Un total de 2534 morues a été échantillonné sur 22 sites de ponte. Pour les deux marqueurs génétiques utilisés, la majorité des valeurs significatives de F ST par paires de populations est due à des comparaisons nord-est et sud-ouest. Une analyse multidimensionnelle basée sur les F ST par paires de populations, une analyse spatiale hiérarchique de variance molécu-laire (SAMOVA) et une analyse hiérarchique de variance moléculaire (AMOVA), réalisées sur les deux types de marqueurs génétiques, révèlent une différenciation significative entre les échantillons du nord-est et du sud-ouest. La recherche de discontinuités génétiques indique la présence de deux barrières au flux génique correspondant aux principaux fronts de courants océaniques localisés dans le nord-ouest et le sud-est du pays aux points où les masses d'eaux chaudes et froides entrent en contact. L'analyse AMOVA détecte aussi des différences génétiques en fonction de la profondeur. Une analyse approfondie montre que les hétérozygoties observées et attendues des microsatellites diminuent significativement avec la profondeur, alors que la fréquence de l'allèle B du locus Pan I augmente. Une expérience de « capture-recapture » réalisée sur les sites de pontes en même temps que les études génétiques révèle que des individus marqués et relâchés dans la région du sud-ouest migrent rarement vers la région du nord-est et vice versa, ce qui confirme que les population...

Most studies of the genetic structure of Atlantic cod have focused on small geographical scales. In the present study, the genetic structure of cod sampled on spawning grounds in the North Atlantic was examined using eight microsatellite loci and the Pan I locus. A total of 954 cod was collected from nine different regions: the Baltic Sea, the North Sea, the Celtic Sea, the Irish Sea and Icelandic waters during spring 2002 and spring 2003, from Norwegian waters and the Faroe Islands (North and West spawning grounds) in spring 2003, and from Canadian waters in 1998. Temporal stability among spawning grounds was observed in Icelandic waters and the Celtic Sea, and no significant difference was observed between the samples from the Baltic Sea and between the samples from Faroese waters. F‐statistics showed significant differences between most populations and a pattern of isolation‐by‐distance was described with microsatellite loci. The Pan I locus revealed the presence of two genetically distinguishable basins, the North‐west Atlantic composed of the Icelandic and Canadian samples and the North‐east Atlantic composed of all other samples. Permutation of allele sizes at each microsatellite locus among allelic states supported a mutational component to the genetic differentiation, indicating a historical origin of the observed variation. Estimation of the time of divergence was approximately 3000 generations, which places the origin of current genetic pattern of cod in the North Atlantic in the late Weichselian (Wisconsinian period), at last glacial maximum. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94, 315–329.

Nine out of 22 microsatellite primers tested were successfully amplified on three samples of cod Gadus morhua L. (two contemporary and one archived otolith samples). All loci were polymorphic (5-23 alleles/locus). The average observed heterozygosity across loci and samples was 0.625, ranging from 0.294 to 0.895 at each locus. All loci were under HardyWeinberg equilibrium, except PGmo56 that showed significant excess of heterozygotes in all studied samples. The isolated loci were suitable for degraded DNA and therefore useful for conducting a long-term temporal study with DNA obtained from archived otoliths of cod.

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