Ribosomal RNA genes are interspersed throughout a heterochromatic chromosome arm in Atlantic salmon.

The ribosomal RNA genes (rDNA) have been mapped by fluorescent in situ hybridization (FISH) using four rDNA probes (rDNA/FISH) to Atlantic salmon chromosomes. Signals appeared over the whole heterochromatic chromosome arm displaying the secondary constriction and satellites. The size polymorphism of this sort arm, revealed by C-banding, was confirmed by rDNA/FISH, supporting large interindividual differences in the number of rDNA copies. Conventional techniques for the detection of nucleolar organizer regions are discussed, and their results are compared with those of rDNA/FISH.