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Retention of juveniles within a hybrid zone between North Sea and Baltic Sea Atlantic cod (Gadus morhua)

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Elucidating the relative roles of dispersal and retention of juvenile stages is an important issue for understanding population structure and evolution in marine organisms. We investigated the genetic population structure of juvenile Atlantic cod (Gadus morhua) within the transition zone between the North Sea and the Baltic Sea, employing nine microsatellite loci, and compared our data with adult cod data from the same area. Small but statistically significant overall differentiation (Fst = 0.003) was found among juvenile samples. Samples of juveniles grouped genetically with adult samples from the same geographical regions. Individual admixture analysis of a large sample of juveniles taken within the transition zone showed that the patterns of genetic differentiation could not be explained by mixing of pure North Sea and Baltic Sea individuals. Instead, the high number of juveniles with intermediate genotypes was compatible with a scenario of exclusive local (transition zone) origin. The results support the hypothesis that population structure in marine fishes is maintained by the retention of juveniles.

L’évaluation des rôles relatifs de la dispersion et de la rétention des stades juvéniles est une question importante pour la compréhension de la structure démographique et de l’évolution des organismes marins. Nous avons examiné la structure génétique de population de jeunes morues franches (Gadus morhua) dans la zone de transition entre la mer du Nord et la Baltique par l’étude de neuf locus microsatellites et nous avons comparé les résultats à ceux obtenus chez les morues adultes de la même région. Il existe une différentiation globale faible (Fst = 0,003), mais statistiquement significative, dans les échantillons de jeunes. Les échantillons de jeunes se regroupent génétiquement avec les échantillons d’adultes de la même région. Une analyse de mélange individuelle d’un grand échantillon de jeunes capturés dans la zone de transition montre que les patrons de différentiation génétique ne peuvent s’expliquer par un mélange d’individus purs de la mer du Nord et de la Baltique. Au contraire, le nombre élevé de jeunes à génotype intermédiaire est compatible avec un scénario d’origine exclusivement locale (zone de transition). Nos résultats appuient l’hypothèse selon laquelle la structure démographique des poissons marins est maintenue par la rétention des jeunes.

Document Type: Research Article

Publication date: October 1, 2005

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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