Rapid phenotypic change and diversification of a soil bacterium during 1000 generations of experimental evolution

Evolutionary pathways open to even relatively simple organisms, such as bac teria, may lead to complex and unpredictable phenotypic changes, both adapt ive and non-adaptive. The evolutionary pathways taken by 18 populations of Ralstonia strain TFD41 while they evolved in defined environments for 1000 generations were examined. Twelve populations evolved in liquid media, whil e six others evolved on agar surfaces. Phenotypic analyses of these derived populations identified some changes that were consistent across all popula tions and others that differed among them. The evolved populations all exhi bited morphological changes in their cell envelopes, including reductions o f the capsule in each population and reduced prostheca-like surface structu res in most populations. Mean cell length increased in most populations (in one case by more than fourfold), although a few populations evolved shorte r cells. Carbon utilization profiles were variable among the evolved popula tions, but two distinct patterns were correlated with genetic markers intro duced at the outset of the experiment. Fatty acid methyl ester composition was less variable across populations, but distinct patterns were correlated with the two physical environments. All 18 populations evolved greatly inc reased sensitivity to bile salts, and all but one had increased adhesion to sand; both patterns consistent with changes in the outer envelope. This ph enotypic diversity contrasts with the fairly uniform increases in competiti ve fitness observed in all populations. This diversity may represent a set of equally probable adaptive solutions to the selective environment; it may also arise from the chance fixation of non-adaptive mutations that hitchhi ked with a more limited set of beneficial mutations.