Molecular-cytogenetic studies of ribosomal RNA genes and heterochromatin in three European Fraxinus species

Three European representatives of the genus Fraxinus were studied for the first time for their rDNA and heterochromatin patterns. The physical mapping of two rRNA gene families 5S and 18S-5.8S-26S (45S) and the distributional pattern of GC-rich regions in the chromosomes have been established by means of fluorescence in situ hybridization (FISH) and fluorochrome banding with chromomycin A(3). The genome size was assessed by flow cytometry. Heterochromatin and rDNA organization was conserved and almost identical for two species from Fraxinus section (F. angustifolia and F. excelsior). The number and position of rDNA loci in F. ornus (section Ornus) were similar; however, the organization of genes was quite different. In this species the 5S and 45S rRNA genes were colocalized at the level of satellites of two chromosome pairs bearing nucleolar organizing regions (NORs). One 5S locus was also observed under the 45S one of one chromosome pair. In F. angustifolia and F. excelsior, only 45S loci were situated at the level of satellites and secondary constrictions, while 5S was located just under 45S in the distal part of the short arm of one out of two marked pairs. The number and position of GC-rich DNA correspond to those of 45S loci. The genome size (2C value) was of 1.54 and 1.68 pg for F. angustifolia and F. excelsior, respectively. Fraxinus ornus possessed the highest 2C DNA value (1.98 pg). In the light of these cytogenetic features the clear differentiation between two sections (Fraxinus and Ornus) was observed both at the rDNA and genome size levels.