Natural hybridisation in birch: triploid hybrids between Betula nana and B. pubescens

Hybridisation between diploid (2n=28) dwarf birch Betula nana L. and tetraploid (2n=56) downy birch B. pubescens Ehrh. has occurred in natural populations in Iceland. About 10% of birch plants randomly collected are triploid (2n=42) hybrids. Ribosomal gene mapping on chromosomes and genomic in situ hybridisation confirms the hybridity. However, the triploid hybrids are not morphologically distinct, i.e. they are not different from diploid and tetraploid birch plants that have intermediate morphology. The triploid hybrids have evidently played an important role in driving bi-directional gene flow between these two species. This paper reviews the extent of interspecific hybridisation in selected birch woodland populations and discusses the significance of natural hybridisation and introgression in birch.