Inducible Mutability and Components of Adaptation Figure 3. Relative fitness of individual population samples from UVR lineages 25-32. Relative fitness was measured by direct competition with the ancestor under (A) non-UVR and (B) UVR conditions. Population samples contained ,108 cells of P. cichorii 302959 after 250 (open bars) and 500 (shaded bars) generations of experimental evolution. Fitness values are means and error bars represent standard error of the mean. Values are significant by two-tailed independent t-test (d.f. = 5, a = 0.05) where indicated (* P,0.05, ** P,0.01, *** P,0.001). P-paired values correspond to two- tailed paired t-tests (d.f. = 5, a = 0.05) between relative fitness values at generation 250 and 500. doi:10.1371/journal.pone.0015975.g003 tions (all P,0.020) with the exception of the isolate from lineage 37 (Figure 6A). Isolates from non-UVR lineages at generation 250 also exhibited a 19% average improvement in relative fitness under UVR conditions (all P,0.006, Figure 6B). Only the relative fitness of the isolate from lineage 36 varied significantly between the two conditions (P = 0.029). When compared to isolates from generation 500, there was no significant difference in relative fitness under non-UVR conditions (Figure 6A) and only isolates from non-UVR lineages 36 and 39 differed significantly from their generation 500 counterparts under UVR conditions (Figure 5B). Growth dynamics Growth parameters were examined to elucidate components of adaptation observed in evolved isolates and population samples in an attempt to determine mechanisms responsible for specific changes in relative fitness. Growth curves of each population sample and single- colony isolate from generations 250 and 500 were plotted to calculate changes in doubling and lag times under both non-UVR and UVR conditions. Isolates and population samples from all evolved lineages displayed improvements in both doubling and lag times. The ancestral P. cichorii 302959 strain exhibited doubling times of 1.4560.09 hours and 1.6860.10 hours at 28uC under non-UVR and UVR conditions, respectively. Population samples from UVR lineages improved their doubling time under non-UVR conditions by an average reduction of 15% at generation 250 and 17% at generation 500 (Figure 7A and B). Doubling times of population samples were further reduced under UVR conditions by an average of 24% and 37% at generations 250 and 500, respectively (Figure 7A and B). Isolates from UVR lineages, including both R and F morphotypes, exhibited similar patterns of doubling time improve- ment, with greater average reductions under UVR conditions at both generation 250 and 500 (Figure 7A and B). However, differences in doubling time improvements under the two conditions were only significant at generation 500 for both population samples and isolates from UVR lineages. Conversely, the average doubling time of isolates from non-UVR lineages was more analogous under non-UVR and UVR conditions and reductions were only significant at generation 500 (Figure 7A and B). Similar improvements in lag time (i.e. reductions in lag time) were also observed. The ancestral P. cichorii 302959 strain exhibited analogous lag times of 10.2760.32 hours and 10.4760.26 hours under non-UVR and UVR conditions, respec- tively. Population samples from UVR lineages improved their lag times under non-UVR conditions by an average reduction of 16% PLoS ONE | www.plosone.org 5 January 2011 | Volume 6 | Issue 1 | e15975