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Genetics, Vol. 179, 511-516, May 2008, Copyright © 2008
doi:10.1534/genetics.107.085621
The Effect of Chromosome Geometry on Genetic Diversity
Pradeep Reddy Marri*,1,
Leigh K. Harris*,
Kathryn Houmiel
,
,
Steven C. Slater and
Howard Ochman*
* Department of Biochemistry and Molecular Biophysics, University of Arizona, Tucson, Arizona 85721, Department of Applied Biological Sciences and The Biodesign Institute, Arizona State University, Mesa, Arizona 85212 and Department of Biology, Seattle Pacific University, Seattle, Washington 98119
1 Corresponding author: BIO5 Institute, Thomas W. Keating Bldg., 1657 E. Helen St., University of Arizona, Tucson, AZ 85721.
E-mail: pradeepm{at}email.arizona.edu
Although organisms with linear chromosomes must solve the problem of fully replicating their chromosome ends, this chromosome configuration has emerged repeatedly during bacterial evolution and is evident in three divergent bacterial phyla. The benefit usually ascribed to this topology is the ability to boost genetic variation through increased recombination. But because numerous processes can impact linkage disequilibrium, such an effect is difficult to assess by comparing across bacterial taxa that possess different chromosome topologies. To test directly the contribution of chromosome architecture to genetic diversity and recombination, we examined sequence variation in strains of Agrobacterium Biovar 1, which are unique among sequenced bacteria in having both a circular and a linear chromosome. Whereas the allelic diversity among strains is generated principally by mutations, intragenic recombination is higher within genes situated on the circular chromosome. In contrast, recombination between genes is, on average, higher on the linear chromosome, but it occurs at the same rate as that observed between genes mapping to the distal portion of the circular chromosome. Collectively, our findings indicate that chromosome topology does not contribute significantly to either allelic or genotypic diversity and that the evolution of linear chromosomes is not based on a facility to recombine.