DNA repair and the evolution of transformation in Haemophilus influenzae.

Under certain environmental conditions, naturally transforming bacteria are induced to pick up DNA released into the environment by other cells of the same or closely related species and, by homologous recombination, integrate that DNA into their chromosome. The selective pressures responsible for the evolution and maintenance of this form of genetic outcrossing, or sex, in bacteria are not known. A prominent hypothesis is that transformation, and sex in general, evolved as a means of obtaining DNA templates to repair damaged regions of the chromosome. Previous results obtained with Bacillus subtilis were consistent with the repair hypothesis. In an effort to explore the generality of those results, I have tested the repair hypothesis with Haemophilus influenzae, a naturally transforming, gram-negative species of bacteria. The results of UV damage-survivorship experiments with H. influenzae were also consistent with that hypothesis. However, additional experiments demonstrate that the higher survival of transformed cultures cannot be accounted for by use of the transforming DNA as templates for repair. I consider alternative hypotheses for the means by which transformation can increase cell survival following UV exposure and discuss the implications of these results with respect to the DNA repair hypothesis and the evolution of transformation.