Growth Parameter Components of Adaptive Specificity during Experimental Evolution of the UVR-Inducible Mutator Pseudomonas cichorii 302959 Michael R. Weigand 1 , Vinh N. Tran 2 , George W. Sundin1,2,3* 1 Program in Genetics, Michigan State University, East Lansing, Michigan, United States of America, 2 Department of Plant Pathology, Michigan State University, East Lansing, Michigan, United States of America, 3 Centers for Microbial Ecology and Pathogenesis, Michigan State University, East Lansing, Michigan, United States of America Abstract Background: Mutagenic DNA repair (MDR) transiently increases mutation rate through the activation of low-fidelity repair polymerases in response to specific, DNA-damaging environmental stress conditions such as ultraviolet radiation (UVR) exposure. These repair polymerases also confer UVR tolerance, intimately linking mutability and survival in bacteria that colone habitats subject to regular UVR exposure. Methodology/Principal Findings: Here, we investigate adaptive specificity in experimental lineages of the highly UVR- mutable epiphytic plant pathogen Pseudomonas cichorii 302959. Relative fitness measurements of isolates and population samples from replicate lineages indicated that adaptive improvements emerged early in all lineages of our evolution experiment and specific increases in relative fitness correlated with distinct improvements in doubling and lag times. Adaptive improvements gained under UVR and non-UVR conditions were acquired preferentially, and differentially contributed to relative fitness under varied growth conditions. Conclusions: These results support our earlier observations that MDR activation may contribute to gains in relative fitness without impeding normal patterns of adaptive specificity in P. cichorii 302959. Citation: Weigand MR, Tran VN, Sundin GW (2011) Growth Parameter Components of Adaptive Specificity during Experimental Evolution of the UVR-Inducible Mutator Pseudomonas cichorii 302959. PLoS ONE 6(1): e15975. doi:10.1371/journal.pone.0015975 Editor: Michael Otto, Laboratory of Human Bacterial Pathogenesis, National Institutes of Health, United States of America Received September 21, 2010; Accepted December 7, 2010; Published January 14, 2011 Copyright: 2011 Weigand et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was funded by the Michigan Agricultural Experiment Station. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: sundin@msu.edu Introduction Mutation is the most fundamental source of variation on which natural selection may drive evolution. However, the predomi- nantly deleterious nature of mutation maintains selection pressure in favor of lower mutation rates as evidenced by the conservation of multiple DNA error-avoidance and error-repair processes [1]. Yet hypermutator (or mutator) strains of bacteria have been observed in a variety of clinical, environmental, and laboratory populations with mutation rates 100-1000 times greater than wild- type due to defects in DNA proofreading and repair machinery [2]. The widespread existence of both constitutive and inducible mutator genotypes suggests that evolutionary strategies of bacteria include mechanisms for increasing mutability [3,4]. These mutator genotypes have not been shown to confer an intrinsic fitness advantage and their abundance within natural populations likely results from hitchhiking with secondary, beneficial mutations which counterbalance the cost of accumulating deleterious mutations [5,6]. Inducible mutability in the form of mutagenic DNA repair (MDR) transiently increases mutation rate through the activation of low-fidelity repair polymerases in response to specific environ- mental stress conditions such as ultraviolet radiation (UVR) exposure. UVR directly damages DNA causing lesions, including cyclobutane pyrimidine dimers and 6-4 photoproducts, that distort the helical structure, interrupting replication fork progression and inducing the SOS response. The SOS regulon coordinates the control of more than 40 unlinked genes involved in DNA repair, recombination, and cell cycle control [7,8]. Among these, the Y family DNA polymerases polIV (Escherichia coli DinB) and polV (E. coli UmuDC) rescue stalled replication forks and permit DNA synthesis across damaged regions of DNA in a template- independent manner by nature of their high processivity and low fidelity [9]. Secondary mutations caused by these error-prone polymerases comprise the majority of sequence alterations derived from UVR exposure and define the inducible mutator phenotype. This translesion DNA synthesis activity confers both UVR tolerance and inducible mutability, intimately linking survival and mutation in habitats subject to regular UVR exposure. MDR-mediated UVR tolerance provides a critical ecological advantage to epiphytic plant pathogens harboring Y family polymerases that reside in habitats optimized for solar UVR exposure [10,11]. We have previously reported our studies of experimental evolution with the highly UVR-mutable celery pathogen Pseudomonas cichorii 302959 [12]. The major contributor of UVR-induced mutability in this strain is rulAB, a homolog of PLoS ONE | www.plosone.org 1 January 2011 | Volume 6 | Issue 1 | e15975