Beneficial mutations are of high interest to understand adaptation processes. However, because beneficial
mutations are very rare, very few experimental data exist. In this proposal we will take advantage of a
recently developed marker system, which allows the tracing of cells that have obtained a beneficial mutation.
Using this system, the mutation rate and selection coefficients of beneficial mutations will be determined.
Furthermore, the adaptive landscape will be investigated by comparing parallel cultures. Finally, expression
profiling and indel mapping will be used to identify the genetic changes that are causing the selective advantage.
We characterized the evolutionary dynamics of an extra chromosomal element (plasmid) in the bacterium
Escherichia coli. We showed that the plasmid imposes a cost of carriage on the host cell, but during our
experimental evolution experiments this cost could be ameliorated by mutations in the plasmid. Most important, we
also showed that the cost of carriage differs among host genotypes, which also have a very host specific change in
gene expression in response to the presence of the plasmid. Furthermore, we also show that the plasmid affects
microsatellite stability in E. coli, resulting in a higher microsatellite mutation rate in the presence of the plasmid.