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Introgression and phylogeography of Betula nana (diploid), B. pubescens (tetraploid) and their triploid hybrids in Iceland inferred from cpDNA haplotype variation

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Abstract:

Abstract Aim 

The objective was to find direct genetic evidence supporting introgressive hybridization between tetraploid tree birch (Betula pubescens) and diploid dwarf birch (B. nana), via triploid hybrids, and to investigate an association between the introgression and phylogeographical distribution of Icelandic birch. Location 

Samples were collected from 463 trees in 12 woodlands in Iceland and eight locations in Norway, Sweden, Scotland and Greenland. Methods 

Ploidy status of individual trees was determined by chromosome counting. Variation in the chloroplast genome was assessed using polymerase chain reaction-restriction fragment length polymorphism. The geographical distribution of the haplotypes was mapped. The haplotype variation and introgression ratios (IG) were analysed statistically. Results 

Thirteen haplotypes were identified among Icelandic samples. The most common haplotype (T, 49% occurrence) was present in all ploidy groups and in all woodlands. All common haplotypes were shared between the triploid group and the parental species, indicating introgressive hybridization. This was supported by the statistical analysis of IG indices and the variation components. Considerable differences existed among samples, shaped by isolation by distance and local introgression. An east–west phylogeographical distribution in Iceland was observed. Main conclusions 

Despite extensive introgression across species and ploidy levels, a biogeographical pattern has been observed, and this may indicate different population histories or multiple origins of Icelandic birch. The chloroplast haplotype diversity found in Iceland resembles that found in birch populations from northern Scandinavia.

Keywords: Betula; Iceland; birch; chloroplast haplotypes; hybridization; introgression; phylogeography; polyploidy; triploid hybrid

Document Type: Research Article

DOI: http://dx.doi.org/10.1111/j.1365-2699.2010.02353.x

Affiliations: Faculty of Life and Environmental Sciences, School of Engineering and Natural Sciences, University of Iceland, Askja - Stugata 7, Reykjavík, IS-101, Iceland

Publication date: November 1, 2010

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