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Open Access Research article

The expansion of amino-acid repeats is not associated to adaptive evolution in mammalian genes

Fernando Cruz12*, Julien Roux12 and Marc Robinson-Rechavi12

Author Affiliations

1 Department of Ecology and Evolution, Biophore, University of Lausanne, 1015 Lausanne, Switzerland

2 Swiss Institute of Bioinformatics, CH-1015 Lausanne, Switzerland

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BMC Genomics 2009, 10:619  doi:10.1186/1471-2164-10-619

Published: 18 December 2009



The expansion of amino acid repeats is determined by a high mutation rate and can be increased or limited by selection. It has been suggested that recent expansions could be associated with the potential of adaptation to new environments. In this work, we quantify the strength of this association, as well as the contribution of potential confounding factors.


Mammalian positively selected genes have accumulated more recent amino acid repeats than other mammalian genes. However, we found little support for an accelerated evolutionary rate as the main driver for the expansion of amino acid repeats. The most significant predictors of amino acid repeats are gene function and GC content. There is no correlation with expression level.


Our analyses show that amino acid repeat expansions are causally independent from protein adaptive evolution in mammalian genomes. Relaxed purifying selection or positive selection do not associate with more or more recent amino acid repeats. Their occurrence is slightly favoured by the sequence context but mainly determined by the molecular function of the gene.