The genus Betula is distributed throughout the northern temperate regions. In most of Europe there are two species of tree birch (Atkinson, 1992): silver birch (Betula pendula Roth., diploid 2n = 2x = 28) and downy birch (Betula pubescens Ehrh., tetraploid 2n = 4x = 56). There is considerable overlap in the distribution of these two species, but silver birch extends further south into the Balkans, while downy birch is found in northern Europe, including Fennoscandia (the northern part of Scandinavia and Finland). Another birch species found in Europe is dwarf birch (Betula nana L., diploid 2n = 2x = 28) (de Groot et al., 1997). It is a circumpolar species with a distribution in both Alpine and Arctic regions that overlaps with the range of tree birch species, especially with downy birch in northern Scandinavia and Russia. Only dwarf birch 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.

Hybridization between B. nana and B. pubescens is widespread in Iceland. The species can be distinguished from each other morphologically, and from the triploid hybrids. The overlapping morphological variation indicates bidirectional introgression between the two species via triploid hybrids. Iceland could be considered a birch hybrid zone, harbouring genetic variation which may be advantageous in subarctic regions.

Two birch species coexist in Iceland, dwarf birch Betula nana and tree birch B. pubescens. Both species are variable morphologically, which has been shown to be due to introgressive hybridization via interspecific hybrids. The aim of this study was to examine if the introgression could be related to genome size. We characterized 42 plants from Bifröst woodland morphologically and cytogenetically. The population consisted of diploid B. nana (38%), tetraploid B. pubescens (55%), and triploid hybrids (7%). Genome size was measured from 12 plants, using Feulgen DNA image densitometry (FDM) on spring leaf buds and flow cytometry (FCM) with dormant winter twigs. The use of winter twigs for FCM is novel. The average 1C-values for diploid, triploid, and tetraploid plants were 448, 666, and 882 Mbp, respectively. Monoploid genome sizes were found to be statistically constant among ploidy levels. This stability is in contrast to the different taxonomic positions of the di-and tetraploids and also contrasts with the frequent occurrence of genome downsizing in polyploids.

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