parallel evolution

Cooper TF, Rozen DE & Lenski RE 2003 Parallel changes in gene expression after 20,000 generations of evolution in Escherichia coli. PNAS 100:1072-1077.

• the expression of 59 genes had changed significantly
• all 59 were changed in the same direction relative to the ancestor
• many of these genes were members of the cAMP-cAMP receptor protein (CRP) and guanosine tetraphosphate (ppGpp) regulons

• sequencing of several genes controlling the effectors of these regulons found a nonsynonymous mutation in spoT in one population
• moving this mutation into the ancestral background showed that it increased fitness and produced many of the expression changes manifest after 20,000 generations
• no effect on fitness when introduced into the other evolved population
• indicating that a mutation of similar effect was present already

• the repeated finding of the same or similar change in multiple lineages is a strong indicator that it is adaptive

• the fitness trajectories of the 12 populations were similar, but it is unclear to what extent this parallelism extends to the underlying physiology and genetics
• we address this issue using macroarrays to evaluate the extent of parallel expression changes in two of these populations