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      Finely Orchestrated Movements: Evolution of the Ribosomal RNA Genes

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      Genetics

      Genetics Society of America

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          Abstract

          Evolution of the tandemly repeated ribosomal RNA (rRNA) genes is intriguing because in each species all units within the array are highly uniform in sequence but that sequence differs between species. In this review we summarize the origins of the current models to explain this process of concerted evolution, emphasizing early studies of recombination in yeast and more recent studies in Drosophila and mammalian systems. These studies suggest that unequal crossover is the major driving force in the evolution of the rRNA genes with sister chromatid exchange occurring more often than exchange between homologs. Gene conversion is also believed to play a role; however, direct evidence for its involvement has not been obtained. Remarkably, concerted evolution is so well orchestrated that even transposable elements that insert into a large fraction of the rRNA genes appear to have little effect on the process. Finally, we summarize data that suggest that recombination in the rDNA locus of higher eukaryotes is sufficiently frequent to monitor changes within a few generations.

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          Most cited references 36

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          Concerted and birth-and-death evolution of multigene families.

          Until around 1990, most multigene families were thought to be subject to concerted evolution, in which all member genes of a family evolve as a unit in concert. However, phylogenetic analysis of MHC and other immune system genes showed a quite different evolutionary pattern, and a new model called birth-and-death evolution was proposed. In this model, new genes are created by gene duplication and some duplicate genes stay in the genome for a long time, whereas others are inactivated or deleted from the genome. Later investigations have shown that most non-rRNA genes including highly conserved histone or ubiquitin genes are subject to this type of evolution. However, the controversy over the two models is still continuing because the distinction between the two models becomes difficult when sequence differences are small. Unlike concerted evolution, the model of birth-and-death evolution can give some insights into the origins of new genetic systems or new phenotypic characters.
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            Molecular drive: a cohesive mode of species evolution.

             Lynn Dover (1982)
            It is generally accepted that mutations may become fixed in a population by natural selection and genetic drift. In the case of many families of genes and noncoding sequences, however, fixation of mutations within a population may proceed as a consequence of molecular mechanisms of turnover within the genome. These mechanisms can be both random and directional in activity. There are circumstances in which the unusual concerted pattern of fixation permits the establishment of biological novelty and species discontinuities in a manner not predicted by the classical genetics of natural selection and genetic drift.
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              Repeated genes in eukaryotes.

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                Author and article information

                Journal
                Genetics
                Genetics
                Genetics Society of America
                0016-6731
                1943-2631
                February 22 2007
                February 2007
                February 2007
                February 22 2007
                : 175
                : 2
                : 477-485
                Article
                10.1534/genetics.107.071399
                1800602
                17322354
                © 2007

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