MOLECULAR CYTOGENETICS OF ICELANDIC BIRCH SPECIES - PHYSICAL MAPPING BY IN-SITU HYBRIDIZATION AND RDNA POLYMORPHISM

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 deter mine the chromosomal locations of the major ribosomal gene family (18S -5.8S-26S rDNA) by fluorescent in situ hybridization in two Icelandic birch species, Betula pubescens Ehrh. and Betula nana L. In the tetrap loid birch (B. pubescens), the rDNA was localized on four major and tw o minor sites, while the diploid dwarf birch (B. nana) had four major sites. The major loci in both species were in nucleolus organizer regi ons, close to the centromeres of a pair of metacentric and a pair of s ub-metacentric chromosomes. The dispersed interphase in situ hybridiza tion pattern showed gene expression at all major sites. The two additi onal loci in B. pubescens, when detected, appeared to be sub-telomeric and inactive at interphase. Southern analysis of rDNA showed consider able restriction fragment length polymorphism in B. pubescens. Some po lymorphism 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.