Ribosomal RNA gene loci and silver-stained nucleolar organizer regions associated with heterochromatin in Alaskan char Salvelinus malma and chum salmon Oncorhynchus keta

Nucleolus-forming 5.8S, 18S and 28S ribosomal RNA gene (rDNA) loci were assigned by fluorescence in situ hybridization (FISH) to the distal half of the short arms of a large-sized submetacentric pair in the Alaskan char (Salvelinus malma) and to the distal region of the long arms of a medium-sized submetacentric pair in the chum salmon (Oncorhynchus keta), respectively. In each species, heteromorphic FISH signals, spanning whole satellite region and secondary constriction, imply an intraspecific variation in the size of rDNA loci. Size variation of silver-stained nucleolar organizer regions (AgNORs) was also apparent between or within the assigned rDNA loci in each species, suggesting a possible inter- or intralocus inactivation of rDNAs. C-band positivity of assigned rDNA loci and AgNORs unequivocally showed their association with heterochromatin in these species.