A comparative analysis of NORs in diploid and triploid salmonids: Implications with respect to the diploidization process occurring in this fish group

Abstract

In order to study the way in which the process of genome diploidization involving salmonid fishes affects the ribosomal genes, chromosome location of active nucleolus organizer regions (NORs) was determined in diploid and triploid individuals of two species of salmonids — rainbow trout and coho salmon. These species belong to an aquaculturally important fish group of tètraploid ancestral origin. Diploids of both species show only one NOR-bearing chromosome pair, while new NOR sites, not detected in diploids, are observed in triploid karyotypes. However, the number of active NORs has not increased significantly with respect to the expected for the triploid level because some of the standard diploid NORs are repressed in triploids. Active NORs appear as CMA3-positive and DAPI-negative clusters in diploid and triploid karyotypes of both species, except the M3 type of rainbow trout which was observed only in triploid individuals. The data obtained suggest: (i) that diploidization of NORs still remains incomplete in the species of this group, and (ii) that the response of NOR-bearing chromosomes to fluorochromes can depend on changes between the inactivated and activated NOR-states associated with the triploidization. The variations in the rDNA of the NOR-type M3, which occur during the diploidization process, can account for the absence of response to fluorochrome staining. Other structural and evolutionary aspects regarding NORs are also discussed. © The Genetical Society of Great Britain.