The 5S ribosomal DNA (rDNA) consists of one transcriptional unit of about 120 base pairs, which is separated from the next unit by a non-transcribed spacer (NTS). The coding sequence and the NTS together form a repeat unit which can be found in hundreds to thousands of copies tandemly repeated in the genomes. The NTS regions seem to be subject to rapid evolution. The first general model of evolution of these multigene families was referred to as divergent evolution, based on studies using hemoglobin and myoglobin as model systems. Later studies showed that nucleotide sequences of different multigene family members are more closely related within species than between species. This observation led to a new model of multigene family evolution, termed concerted evolution. Another model of evolution, named the birth-and-death model, has been found to be more suitable to explain the long-term evolution of these multigene families. According to this model, new genes originate by successive duplications, and these new genes are either maintained for a long time or are lost, or else degenerate into pseudogenes. In this review we describe different sources of variability in the 5S rDNA genes observed in several distinct fish species. This variability is mainly referred to NTSs and includes the presence of other multigene families (mainly LINEs, SINEs, non-LTR retrotransposons, and U snRNA families). Different types of microsatellites have also been found to contribute to the increase of variability in this region. Our recent results suggest that horizontal transfer contributes to the increase of diversity in the NTSs of some species. Variability in the 5S rDNA coding region affecting the stability of the structure, but without effects on the function of the 5S rRNA, is also described. Retrotransposons seem to be responsible for the high dynamism of 5S rDNA, while microsatellites acting as recombination hot spots could stabilize a wide variety of unusual DNA structures, affecting DNA replication and enhancing or decreasing promoter activity in gene expression. The relationship between the high variability found at molecular level and the low variability found at chromosomal level is also discussed.

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