MBE Advance Access published online on December 13, 2007
Molecular Biology and Evolution, doi:10.1093/molbev/msm275
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Research Article |
Contrasting the Efficacy of Selection on the X and Autosomes in Drosophila
Department of Molecular Biology and Genetics, Cornell University, Ithaca NY 14853
To whom correspondence should be addressed (Department of Molecular Biology and Genetics, 227 Biotechnology Building, Cornell University, Ithaca NY 14853, aml69{at}cornell.edu)
Received for publication August 31, 2007. Revision received November 16, 2007. Accepted for publication December 7, 2007.
To investigate the relative efficacy of both positive and purifying natural selection on the X chromosome and the autosomes in Drosophila, we compared rates and patterns of molecular evolution between these chromosome sets using the newly available alignments of orthologous genes from twelve species. Parameters that may influence the relative X vs. autosomal substitution rates include the relative effective population sizes, the male and female germline mutation rates, the distribution of allelic effects on fitness, and the degree of dominance of novel mutations. Our analysis reveals that codon usage bias is consistently greater for X-linked genes, suggesting that purifying selection consistently has greater efficacy on the X chromosome than on the autosomes across the Drosophila phylogeny. However, our results are less consistent with respect to the efficacy of positive selection, with only some lineages showing a higher substitution rate on the X chromosome. This suggests that either the distribution of selective effects of mutations or other relevant parameters are sufficiently variable across species to tip the balance in different ways in individual lineages. These data suggest that rates of substitution are not solely governed by adaptive evolution. This genome-wide analysis provides a clear picture that the efficacy of selection varies intragenomically, and that this effect is markedly more consistent across the phylogeny in the case of purifying selection. Our results also suggest that simple models that predict systematic differences in rates of evolution between the X and the autosomes can only be made to be compatible with these Drosophila data if the relevant population genetic parameters that drive substitution rates differ among species and chromosomal contexts.
Key Words: Faster-X • positive selection • purifying selection • Drosophila
# These authors contributed equally to this work