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Phylogeographic structure of brown trout Salmo trutta in Britain and Ireland: glacial refugia, postglacial colonization and origins of sympatric populations

Authors

  • N. J. McKeown,

    1. School of Biological Sciences, Queen's University Belfast, MBC 97 Lisburn Road, Belfast BT9 7BL, Northern Ireland, U.K.
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    • *

      Present address: School of Biological Science, Royal Holloway University of London, Surrey, TW20 0EX, England, U.K.

  • R. A. Hynes,

    1. School of Biological Sciences, Queen's University Belfast, MBC 97 Lisburn Road, Belfast BT9 7BL, Northern Ireland, U.K.
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  • R. A. Duguid,

    1. School of Biological Sciences, Queen's University Belfast, MBC 97 Lisburn Road, Belfast BT9 7BL, Northern Ireland, U.K.
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    • Present address: Scottish Environment Protection Agency, 7 Whitefriars Crescent, Perth, PH2 0 PA, Scotland, U.K.

  • A. Ferguson,

    1. School of Biological Sciences, Queen's University Belfast, MBC 97 Lisburn Road, Belfast BT9 7BL, Northern Ireland, U.K.
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  • P. A. Prodöhl

    Corresponding author
    1. School of Biological Sciences, Queen's University Belfast, MBC 97 Lisburn Road, Belfast BT9 7BL, Northern Ireland, U.K.
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Tel.: + 44 2890 972267; email: p.prodohl@qub.ac.uk

Abstract

The phylogeographical structure of brown trout Salmo trutta in Britain and Ireland was studied using polymerase chain reaction–restriction fragment length polymorphism (PCR–RFLP) analysis of four mitochondrial DNA segments (16S/ND1, ND5/6, COXIII/ND5 and ND5/12S). Analysis of 3636 individuals from 83 sites–morphotypes revealed a total of 25 haplotypes. These haplotypes were nested in seven two-step clades. Although there was a clear geographical patterning to the occurrence of derived clades, admixture among ancestral clades was extensive throughout the studied area. A relevant feature of the data was that some populations contained mixtures of highly divergent clades. This type II phylogeographic pattern is uncommon in nature. Clade intermixing is likely to have taken place during earlier interglacials as well as since the Last Glacial Maximum. The anadromous life history of many S. trutta populations has probably also contributed to clade mixing. Based on the data presented here and published data, postglacial colonization of Britain and Ireland most likely involved S. trutta from at least five potential glacial refuges. Probable locations for such refugia were: south of England–western France, east of the Baltic Sea, western Ireland, Celtic Sea and North Sea. Ferox S. trutta, as defined by their longevity, late maturation and piscivory, exhibited a strong association with a particular clade indicating that they share a common ancestor. Current evidence indicates that the Lough Melvin gillaroo S. trutta and sonaghen S. trutta sympatric types diverged prior to colonization of Lough Melvin and, although limited gene flow has occurred since secondary contact, they have remained largely reproductively isolated due to inlet and outlet river spawning segregation. Gillaroo S. trutta may reflect descendents of a previously more widespread lineage that has declined due to habitat alterations particularly affecting outlet rivers. The mosaic-like distribution of mtDNA lineages means that conservation prioritization in Britain and Ireland should be based on the biological characteristics of local populations rather than solely on evolutionary lineages.

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