Extensive sharing of chloroplast haplotypes among the silver birch, Betula pendula Roth., the downy birch, B. pubescens Ehrh., and the dwarf birch, B. nana L., was discovered using polymerase chain reaction–restriction fragment length polymporphism markers. The geographical component of the genetic variation was stronger than the species component: the species were not significantly different while 11% of the variation could be attributed to differentiation between the two main regions studied, Scandinavia and western Russia. All haplotypes occurring in more than 2% of the individuals were shared among the species and the introgression ratios were quite large: 0.79 between B. pubescens and B. pendula and 0.67 between B. pubescens and B. nana. The data also indicate that B. pendula individuals are more similar to sympatric B. pubescens than to B. pendula individuals from nearby forests. However, this trend is not as pronounced when B. pubescens is considered, suggesting that introgression is not symmetrical. The haplotype sharing among the three Betula species is most likely caused by hybridization and subsequent cytoplasmic introgression.
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