The comparative analysis of NOR polymorphism detected by FISH and Ag-staining on horse chromosomes

The nucleolus organizer regions were investigated by FISH with biotinylated rDNA probe and silver staining on chromosomes of the domestic horse (Equus caballus}. Ribosomal RNA loci were mapped at the secondary constrictions of the short arm of chromosome 1 and at the pericentromeric regions of chromosomes 27, 28 and 31. A new nucleolus-organizing chromosome 27 was identified. The interindividual, and interchromosomal polymorphism of NORs was described in 26 horses from 5 breeds. The relative rRNA gene activity was evaluated by the number of silver-stained chromosomes, and the size of silver deposits. The relative amount of rDNA in NORs was estimated by intensity of fluorescent hybridization signals. The size of silver deposits and intensity of fluorescence after FISH were determined for each NOR using arbitrary scales of 0-3 and 0-5, respectively. The statistical analysis of these data revealed a tendency for differentiation of NOR-bearing chromosome pairs by the rRNA gene number of copies, and the activity. A close correlation between activity of the rRNA genes and the relative number of rRNA gene copies was not found for NORs of homologous chromosomes.