Originally published as Genetics Published Articles Ahead of Print on November 3, 2008.

Genetics, Vol. 181, 199-208, January 2009, Copyright © 2009
doi:10.1534/genetics.108.096131

Long-Term Effects of Inducible Mutagenic DNA Repair on Relative Fitness and Phenotypic Diversification in Pseudomonas cichorii 302959

Michael R. Weigand and George W. Sundin1

Program in Genetics, Department of Plant Pathology, Centers for Microbial Ecology and Pathogenesis, Michigan State University, East Lansing, Michigan 48824

1 Corresponding author: Department of Plant Pathology, Michigan State University, 103 Center for Integrated Plant Systems, East Lansing, MI 48824.
E-mail: sundin{at}msu.edu

Mutagenic DNA repair (MDR) employs low-fidelity DNA polymerases capable of replicating past DNA lesions resulting from exposure to high-energy ultraviolet radiation (UVR). MDR confers UVR tolerance and activation initiates a transient mutator phenotype that may provide opportunities for adaptation. To investigate the potential role of MDR in adaptation, we have propagated parallel lineages of the highly mutable epiphytic plant pathogen Pseudomonas cichorii 302959 with daily UVR activation (UVR lineages) for ~500 generations. Here we examine those lineages through the measurement of relative fitness and observation of distinct colony morphotypes that emerged. Isolates and population samples from UVR lineages displayed gains in fitness relative to the ancestor despite increased rates of inducible mutation to rifampicin resistance. Regular activation of MDR resulted in the maintenance of genetic diversity within UVR lineages, including the reproducible diversification and coexistence of "round" and "fuzzy" colony morphotypes. These results suggest that inducible mutability may present a reasonable strategy for adaptive evolution in stressful environments by contributing to gains in relative fitness and diversification.