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Long-term effective population sizes, temporal stability of genetic composition and potential for local adaptation in anadromous brown trout (Salmo trutta) populations

Authors

  • Michael M. Hansen,

    Corresponding author
    1. Danish Institute for Fisheries Research, Department of Inland Fisheries, Vejlsøvej 39, DK-8600 Silkeborg, Denmark,
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  • Daniel E. Ruzzante,

    1. Danish Institute for Fisheries Research, Department of Inland Fisheries, Vejlsøvej 39, DK-8600 Silkeborg, Denmark,
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    • Present address: Department of Biology, Dalhousie University, Halifax, NS, B3H 4J1, Canada.

  • Einar E. Nielsen,

    1. Danish Institute for Fisheries Research, Department of Inland Fisheries, Vejlsøvej 39, DK-8600 Silkeborg, Denmark,
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  • Dorte Bekkevold,

    1. Danish Institute for Fisheries Research, Department of Inland Fisheries, Vejlsøvej 39, DK-8600 Silkeborg, Denmark,
    2. Department of Ecology and Genetics, University of Aarhus, Ny Munkegade, Building 540, DK-8000 Aarhus C, Denmark
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  • Karen-Lise D. Mensberg

    1. Danish Institute for Fisheries Research, Department of Inland Fisheries, Vejlsøvej 39, DK-8600 Silkeborg, Denmark,
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Michael M. Hansen. Fax: + 45 89 213150; E-mail: mmh@dfu.min.dk

Abstract

We examined the long-term temporal (1910s to 1990s) genetic variation at eight microsatellite DNA loci in brown trout (Salmo trutta L) collected from five anadromous populations in Denmark to assess the long-term stability of genetic composition and to estimate effective population sizes (Ne). Contemporary and historical samples consisted of tissue and archived scales, respectively. Pairwise θST estimates, a hierarchical analysis of molecular variance (amova) and multidimensional scaling analysis of pairwise genetic distances between samples revealed much closer genetic relationships among temporal samples from the same populations than among samples from different populations. Estimates of Ne, using a likelihood-based implementation of the temporal method, revealed Ne ≥ 500 in two of three populations for which we have historical data. A third population in a small (3 km) river showed Ne ≥ 300. Assuming a stepping-stone model of gene flow we considered the relative roles of gene flow, random genetic drift and selection to assess the possibilities for local adaptation. The requirements for local adaptation were fulfilled, but only adaptations resulting from strong selection were expected to occur at the level of individual populations. Adaptations resulting from weak selection were more likely to occur on a regional basis, i.e. encompassing several populations. Ne appears to have declined recently in at least one of the studied populations, and the documented recent declines of many other anadromous brown trout populations may affect the persistence of local adaptation.

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