The extreme Beringian/Atlantic disjunction in Saxifraga rivularis (Saxifragaceae) has formed at least twice

Authors

  • Kristine Bakke Westergaard,

    Corresponding author
    1. Tromsø University Museum, University of Tromsø, NO-9037 Tromsø, Norway
    2. National Centre for Biosystematics, Natural History Museum, University of Oslo, PO Box 1172 Blindern, NO-0318 Oslo, Norway
      Kristine Bakke Westergaard, Tromsø University Museum, University of Tromsø, NO-9037 Tromsø, Norway.
      E-mail: kristine.westergaard@uit.no
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  • Marte Holten Jørgensen,

    1. Department of Biology, Centre for Ecological and Evolutionary Synthesis (CEES), University of Oslo, PO Box 1066 Blindern, NO-0316 Oslo, Norway
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  • Tove M. Gabrielsen,

    1. National Centre for Biosystematics, Natural History Museum, University of Oslo, PO Box 1172 Blindern, NO-0318 Oslo, Norway
    2. University Centre in Svalbard, PO Box 156, NO-9171 Longyearbyen, Norway
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  • Inger Greve Alsos,

    1. University Centre in Svalbard, PO Box 156, NO-9171 Longyearbyen, Norway
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  • Christian Brochmann

    1. National Centre for Biosystematics, Natural History Museum, University of Oslo, PO Box 1172 Blindern, NO-0318 Oslo, Norway
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Kristine Bakke Westergaard, Tromsø University Museum, University of Tromsø, NO-9037 Tromsø, Norway.
E-mail: kristine.westergaard@uit.no

Abstract

Aim  The oceanic Saxifraga rivularis L. presents one of the most extreme disjunctions known in the arctic flora: it has a small amphi-Beringian range and a larger amphi-Atlantic one. It was recently suggested to have had a single allopolyploid origin in Beringia at least one glacial cycle ago, followed by gradual expansion in a more humid period and differentiation into two allopatric subspecies (the Atlantic ssp. rivularis and the Beringian ssp. arctolitoralis). Here we explore the history of its extreme disjunction.

Location  The amphi-Beringian and northern amphi-Atlantic regions.

Methods  We obtained amplified fragment length polymorphisms (AFLPs) and chloroplast DNA sequences from 36 populations (287 individuals) and 13 populations (15 individuals), respectively. The data were analysed using principal coordinates analyses, Bayesian clustering methods, and analyses of molecular variance.

Results  Two distinctly divergent AFLP groups were observed, corresponding to the two described subspecies, but, surprisingly, four of the West Atlantic populations belonged to the supposedly Beringian endemic ssp. arctolitoralis. This was confirmed by re-examination of their morphological characteristics. The overall AFLP diversity in the species was low (26.4% polymorphic markers), and there was no variation in the five investigated chloroplast DNA (cpDNA) regions. There was little geographic structuring of the AFLP diversity within each subspecies, even across the extreme disjunction in ssp. arctolitoralis, across the Bering Sea, and across the Atlantic Ocean, except that most plants from the arctic Svalbard archipelago formed a separate genetic group with relatively high diversity.

Main conclusions  The extreme disjunction in S. rivularis has evidently formed at least twice. The first expansion from Beringia was followed by allopatric differentiation into one Beringian and one Atlantic subspecies, which are distinctly divergent at AFLP loci but still harbour identical cpDNA haplotypes, suggesting that the expansion was quite recent but before the last glaciation. The next expansion from Beringia probably occurred by means of several long-distance dispersals in the current interglacial, resulting in the colonization of the western Atlantic region by ssp. arctolitoralis. The poor geographic structuring within each subspecies suggests frequent long-distance dispersals from two main Weichselian refugia, one Beringian and one western-central European, but it is possible that the genetic group in Svalbard originates from an additional refugium.

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