Molecular cytogenetics of Icelandic birch species: physical mapping by in situ hybridization and rDNA polymorphism

Abstract

The physical mapping of genes can reveal the organization of a genome and identify relationships of plant species, especially where they are involved in interspecific hybridization and polyploidy. Here we determine the chromosomal locations of the major ribosomal gene family (18S-5.8S-26S rDNA) by fluorescent in situ hybridization in two Icelandic birch species, Betulapubescens Ehrh. and Betulanana L. In the tetraploid birch (B. pubescens), the rDNA was localized on four major and two minor sites, while the diploid dwarf birch (B. nana) had four major sites. The major loci in both species were in nucleolus organizer regions, close to the centromeres of a pair of metacentric and a pair of sub-metacentric chromosomes. The dispersed interphase in situ hybridization pattern showed gene expression at all major sites. The two additional loci in B. pubescens, when detected, appeared to be sub-telomeric and inactive at interphase. Southern analysis of rDNA showed considerable restriction fragment length polymorphism in B. pubescens. Some polymorphism may reflect gene flow among populations and between the two co-existing birch species. The understanding of genome relationships, gene introgression, and evolution of birch species will be important to the breeding programmes steered towards environmental conservation and forestry.