Genome differentiation in Aegilops. 2. Physical mapping of 5S and 18S-26S ribosomal RNA gene families in diploid species.

The distribution of the 5S and 18S-5.8S-26S (18S-26S) ribosomal RNA (rRNA) gene families on chromosomes of all diploid Aegilops species was studied by in situ hybridization with pTa71 (18S-26S rDNA) and pTa794 (5S rDNA) DNA clones. One major 18S-26S rDNA locus was found in the nucleolus organizer region (NOR) of each of the species Aegilops tauschii and Aegilops uniaristata and two loci were detected in the remaining species. In addition to major NORs, from one to nine minor loci were observed; their numbers and chromosomal locations were species-specific. Some minor loci were polymorphic, whereas others were conserved. One or two 5S rDNA loci were observed in the short arms of the chromosomes of groups 1 and 5 of all diploid Aegilops species except Ae. uniaristata, where one 5S rDNA site was located in the distal part of the long arm of chromosome 1N. The 5S rDNA loci were not associated with NORs; however, the relative positions of two ribosomal RNA gene families were diagnostic for chromosomes of homoeologous groups 1, 5, and 6. Implications of these results for establishing phylogenetic relationships of diploid Aegilops species and mechanisms of genome differentiation are discussed. Key words : wheat, Triticum, Aegilops, 5S rRNA, 18S-26S rRNA, in situ hybridization, evolution.