The phylogeographic architecture of the fucoid seaweed Ascophyllum nodosum: an intertidal ‘marine tree’ and survivor of more than one glacial–interglacial cycle

Authors

  • Jeanine L. Olsen,

    Corresponding author
    1. Department of Marine Benthic Ecology & Evolution, Centre for Ecological and Evolutionary Studies, The University of Groningen, Biological Centre, Kerklaan 30, 9750 AA Haren, The Netherlands
      Jeanine Olsen, Department of Marine Benthic Ecology & Evolution, Centre for Ecological and Evolutionary Studies, The University of Groningen, Biological Centre, Kerklaan 30, 9750 AA Haren, The Netherlands.
      E-mail: j.l.olsen@rug.nl
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  • Frederick W. Zechman,

    1. Department of Biology, California State University at Fresno, 2555 East San Ramon Ave, Fresno, CA 93740, USA
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  • Galice Hoarau,

    1. Department of Marine Benthic Ecology & Evolution, Centre for Ecological and Evolutionary Studies, The University of Groningen, Biological Centre, Kerklaan 30, 9750 AA Haren, The Netherlands
    2. Faculty of Biosciences and Aquaculture, Bodø University College, 8049 Bodø, Norway
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  • James A. Coyer,

    1. Department of Marine Benthic Ecology & Evolution, Centre for Ecological and Evolutionary Studies, The University of Groningen, Biological Centre, Kerklaan 30, 9750 AA Haren, The Netherlands
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  • Wytze T. Stam,

    1. Department of Marine Benthic Ecology & Evolution, Centre for Ecological and Evolutionary Studies, The University of Groningen, Biological Centre, Kerklaan 30, 9750 AA Haren, The Netherlands
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  • Myriam Valero,

    1. Evolution et Genetique des Populations Marines, UMR-CNRS-UPMC 7144, Station Biologique de Roscoff, BP 74, Place Georges Teissier, 29682 Roscoff Cedex, France
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  • Per Åberg

    1. Department of Marine Ecology, University of Gothenburg, Box 461, SE-405 30 Göteborg, Sweden
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Jeanine Olsen, Department of Marine Benthic Ecology & Evolution, Centre for Ecological and Evolutionary Studies, The University of Groningen, Biological Centre, Kerklaan 30, 9750 AA Haren, The Netherlands.
E-mail: j.l.olsen@rug.nl

Abstract

Aim Ascophyllum nodosum (L.) Le Jolis is a dominant fucoid seaweed occurring along sheltered, rocky shores throughout the North Atlantic (but not in the Pacific), where it is a foundational species of the intertidal community. Its large size and vulnerability to ice-scour have led to the hypothesis that contemporary populations in the north-west Atlantic may be the result of de novo recolonization from the north-east Atlantic since the Last Glacial Maximum (LGM) (c. 20 ka). We tested this hypothesis.

Location  Temperate North Atlantic rocky intertidal between c. 42 and 65° N latitude.

Methods  More than 1300 individuals from 28 populations were sampled from across the entire range of A. nodosum and genotyped for six microsatellite loci, and > 500 individuals were genotyped for two mitochondrial loci, an intergenic spacer (IGS) and the tRNA (W) gene (trnW). Population structure and historical demography were analysed in a standard population genetics and coalescence framework.

Results  Based on the presence of private alleles and haplotypes, we found that A. nodosum has survived on both sides of the Atlantic (since before the LGM, dating back to at least the penultimate Eemian interglacial) with similar effective population sizes and divergence times (1.2 and 0.8 Ma). Dispersal has been predominantly from Europe to North America, and there is very weak present-day population differentiation across the North Atlantic. Diversity measures provided additional support for determining the location of refugia.

Main conclusions Ascophyllum nodosum was apparently little affected by the LGM, although contemporary climate change is likely to have major effects on its latitudinal distribution on both sides of the North Atlantic. It is a very long-lived species, analogous in virtually all demographic aspects to a tree – resistant to extinction but vulnerable to catastrophic events. The Brittany peninsula is a hotspot of genetic diversity worthy of conservation.

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