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Journal of Bacteriology, October 2004, p. 6575-6585, Vol. 186, No. 19
0021-9193/04/$08.00+0 DOI: 10.1128/JB.186.19.6575-6585.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Department of Bioengineering and Bioinformatics, Moscow State University,1 Institute for Problems of Information Transmission RAS,4 State Scientific Center GosNIIGenetika, Moscow, Russia,5 Department of Pathology, F. E. Hebert School of Medicine, Uniformed Services University of the Health Sciences,2 National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, Maryland3
Received 7 April 2004/ Accepted 28 June 2004
We describe a simple theoretical framework for identifying orthologous sets of genes that deviate from a clock-like model of evolution. The approach used is based on comparing the evolutionary distances within a set of orthologs to a standard intergenomic distance, which was defined as the median of the distribution of the distances between all one-to-one orthologs. Under the clock-like model, the points on a plot of intergenic distances versus intergenomic distances are expected to fit a straight line. A statistical technique to identify significant deviations from the clock-like behavior is described. For several hundred analyzed orthologous sets representing three well-defined bacterial lineages, the
-Proteobacteria, the
-Proteobacteria, and the Bacillus-Clostridium group, the clock-like null hypothesis could not be rejected for
70% of the sets, whereas the rest showed substantial anomalies. Subsequent detailed phylogenetic analysis of the genes with the strongest deviations indicated that over one-half of these genes probably underwent a distinct form of horizontal gene transfer, xenologous gene displacement, in which a gene is displaced by an ortholog from a different lineage. The remaining deviations from the clock-like model could be explained by lineage-specific acceleration of evolution. The results indicate that although xenologous gene displacement is a major force in bacterial evolution, a significant majority of orthologous gene sets in three major bacterial lineages evolved in accordance with the clock-like model. The approach described here allows rapid detection of deviations from this mode of evolution on the genome scale.
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