R-043. Long-Term Effect of Ultraviolet Radiation Induced Mutagenic DNA Repair on Relative Fitness and Phenotypic Diversification in Pseudomonas cichorii 302959

M. R. Weigand, G. W. Sundin;
Michigan State Univ., East Lansing, MI.

Mutagenic DNA repair (MDR) employs low-fidelity, Y-Family DNA polymerases capable of replicating over DNA lesions resulting from exposure to high energy ultraviolet radiation (UVR). This process, also termed translesion DNA synthesis, confers UVR tolerance. MDR activation initiates a transient mutator phenotype which, similar to general mutators, may provide an evolutionary strategy for bacteria. To investigate the adaptive potential of MDR we have propagated parallel lineages of the highly mutable epiphytic plant pathogen Pseudomonas cichorii 302959 in a serial transfer regime with or without daily UVR irradiation. After 500 generations, changes in relative fitness were measured by direct competition with the common ancestor. Isolates from non-UVR lineages exhibited significant fitness gains indicating that 500 generations of growth by P. cichorii 302959 in this simple environment are adequate to produce measurable and statistically significant changes in relative fitness. Regular activation of MDR by UVR irradiation during long-term experimental evolution did not reduce fitness in P. cichorii 302959 despite the increased mutational load. In contrast, cellular fitness was increased by an average of 15.0% in UVR lineages after 500 generations. Induced mutability in UVR lineages resulted in the reproducible diversification of colony morphotypes and the coexistence of ‘round’ and ‘fuzzy’ morphs that fluctuated stochastically in relative abundance over the course of the experiment. Therefore, inducible mutability may present a reasonable strategy for adaptive evolution generating gains in relative fitness and phenotypic diversification, both advantageous to pathogenic bacteria.