Chromosomal localization and heterochromatin association of ribosomal RNA gene loci and silver-stained nucleolar organizer regions in salmonid fishes

Ribosomal RNA gene (rDNA) loci, including those of nucleolus-forming 18S, 5 .8S and 28S (major) and non-nucleolus-forming 5S (minor) rDNA, were assigne d using fluorescence in situ hybridization (FISH) to the embryonic chromoso mes of rainbow trout (Oncorhynchus mykiss), masu salmon (O. masou), brook t rout (Salvelinus fontinalis) and Japanese huchen (Hucho perryi). In these s pecies, the minor rDNA loci were located basically on 2-4 chromosome pairs, whereas the major rDNA loci were found essentially on one chromosome pair, except for the brook trout. Its major rDNA loci were dispersed on about ha lf of the chromosome complement, showing a considerable interindividual var iation in the number and location. The major and minor rDNA loci were separ ated onto different chromosomes in the examined species, except for the rai nbow trout, in which one chromosome pair had tandemly aligned minor and maj or rDNA loci. Chromosome regions containing both kinds of rDNA loci in each species were found to be stained with C-banding, showing an association of these loci with heterochromatin. Comparison of the assigned major rDNA loc i and sequentially detected silver (Ag)-stained nucleolar organizer regions (AgNORs) in all the species revealed a considerable polymorphism in the nu mber and size of AgNORs among or within those loci, suggesting a possible i nter- or intralocus inactivation of the major rDNAs.