Ribosomal genes in notothenioid fishes: Focus on the chromosomal organisation.

This mini-review makes a survey and a summary of some major issues concerning the chromosomal organisation of ribosomal genes in fish genomes, by using Notothenioidei as the model. The increasing body of information, published during the last two decades on the chromosomal mapping of the two ribosomal genes classes (45S rDNA and 5S rDNA) in notothenioids, makes it possible to recognise the main evolutionary trends across the phylogeny of the group. As one of the major features, the rDNA clusters are organised in a single chromosomal locus in most of the species. This locus is located at different positions along the chromosomes in the basal groups (non-Antarctic Clade), whereas it maintains a strongly conserved location in the cold-adapted species (Antarctic Clade). Important structural changes, leading to the co-localisation of the two ribosomal gene classes, occurred early in the notothenioid phylogeny, perhaps in the common ancestor of the Eleginopidae and Nototheniidae. The cytogenetic evidences indicate that an increased amount of ribosomal genes, organised in two large chromosomal loci, is present in the giant Antarctic fish Dissostichus mawsoni. This gain in rRNA genes is an important genomic change, having possible implications for the fitness of this notothenioid fish that combines large size, pelagic lifestyle and cold-adaptation.