Asymmetric reproductive barriers and mosaic reproductive isolation: insights from Misty lake–stream stickleback

Authors

  • Katja Räsänen,

    Corresponding author
    1. Redpath Museum and Department of Biology, McGill University, Montréal, QC, Canada
    Current affiliation:
    1. Department of Aquatic Ecology, and ETH-Zurich, Institute of Integrative Biology, Duebendorf, Switzerland
    • Correspondence

      Katja Räsänen, Department of Aquatic Ecology, Eawag, Ueberlandstrasse 133, CH-8600 Duebendorf, Switzerland. Tel: +41 58 765 5186; Fax: +41 58 765 5028 E-mail: katja.rasanen@eawag.ch

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  • Andrew P. Hendry

    1. Redpath Museum and Department of Biology, McGill University, Montréal, QC, Canada
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Abstract

Ecological speciation seems to occur readily but is clearly not ubiquitous – and the relative contributions of different reproductive barriers remain unclear in most systems. We here investigate the potential importance of selection against migrants in lake/stream stickleback (Gasterosteus aculeatus) from the Misty Lake system, Canada. This system is of particular interest because one population contrast (Lake vs. Outlet stream) shows very low genetic and morphological divergence, whereas another population contrast (Lake vs. Inlet stream) shows dramatic genetic and morphological divergence apparently without strong and symmetric reproductive barriers. To test whether selection against migrants might solve this “conundrum of missing reproductive isolation”, we performed a fully factorial reciprocal transplant experiment using 225 individually marked stickleback collected from the wild. Relative fitness of the different ecotypes (Lake, Inlet, and Outlet) was assessed based on survival and mass change in experimental enclosures. We found that Inlet fish performed poorly in the lake (selection against migrants in that direction), whereas Lake fish outperformed Inlet fish in all environments (no selection against migrants in the opposite direction). As predicted from their phenotypic and genetic similarity, Outlet and Lake fish performed similarly in all environments. These results suggest that selection against migrants is asymmetric and, together with previous work, indicates that multiple reproductive barriers contribute to reproductive isolation. Similar mosaic patterns of reproductive isolation are likely in other natural systems.

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