Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 102, Issue 11, Pages 4063-4067Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0500436102
Keywords
nearly neutral evolution; synonymous substitution; codon usage; purifying selection; positive selection
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The effect of weak selection driving genome evolution has attracted much attention in the last decade, but the task of measuring the strength of such selection is particularly difficult. A useful approach is to contrast the evolution of X-linked and autosomal genes in two closely related species in a whole-genome analysis. If the fitness effect of mutations is recessive, X-linked genes should evolve more rapidly than autosomal genes when the mutations are advantageous, and they should evolve more slowly than autosomal genes when the mutations are deleterious. We found synonymous substitutions on the X chromosome of human and chimpanzee to be less frequent than those on the autosomes. When calibrated against substitutions in the intergenic regions and pseudogenes to filter out the differences in the mutation rate and ancestral population size between X chromosomes and autosomes, X-linked synonymous substitutions are still 10% less frequent. At least 90% of the synonymous substitutions in human and chimpanzee are estimated to be deleterious, but the fitness effect is weaker than the effect of genetic drift. However, X-linked non-synonymous substitutions are approximate to 30% more frequent than autosomal ones, suggesting the fixation of advantageous mutations that are recessive.
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