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Science 16 January 2009:
Vol. 323. no. 5912, pp. 379 - 382
DOI: 10.1126/science.1167140
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Reports

The Dynamics and Time Scale of Ongoing Genomic Erosion in Symbiotic Bacteria

Nancy A. Moran,1* Heather J. McLaughlin,1 Rotem Sorek2

Among cellular organisms, symbiotic bacteria provide the extreme examples of genome degradation and reduction. However, only isolated snapshots of eroding symbiont genomes have previously been available. We documented the dynamics of symbiont genome evolution by sequencing seven strains of Buchnera aphidicola from pea aphid hosts. We estimated a spontaneous mutation rate of at least 4 x 10–9 substitutions per site per replication, which is more than 10 times as high as the rates previously estimated for any bacteria. We observed a high rate of small insertions and deletions associated with abundant DNA homopolymers, and occasional larger deletions. Although purifying selection eliminates many mutations, some persist, resulting in ongoing loss of genes and DNA from this already tiny genome. Our results provide a general model for the stepwise process leading to genome reduction.

1 Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721, USA.
2 Department of Molecular Genetics, Weizmann Institute of Science, Rehovot 76100, Israel.

* To whom correspondence should be addressed. E-mail: nmoran{at}email.arizona.edu

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Comparative Sequence Analysis of Mycobacterium leprae and the New Leprosy-Causing Mycobacterium lepromatosis.
X. Y. Han, K. C. Sizer, E. J. Thompson, J. Kabanja, J. Li, P. Hu, L. Gomez-Valero, and F. J. Silva (2009)
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Post-Pleistocene radiation of the pea aphid complex revealed by rapidly evolving endosymbionts.
J. Peccoud, J.-C. Simon, H. J. McLaughlin, and N. A. Moran (2009)
PNAS 106, 16315-16320
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