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Abstract
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.