Evolution of Transposable Elements: An IS10 Insertion Increases Fitness in Eschevichia COZ?

Strains of Escherichia coli carrying TnlO, a transposon consisting of two IS10 insertion sequences flanking a segment encoding for a tetracycline-resistance determinant, gain a competitive advantage in chemostat cultures. All TnIO-bearing strains that increase in frequency during competition have a new ISlO insertion that is found in the same location in the genome of those strains. We mapped, by a gradient of transmission, the position of the new ISlO insertion. We examined 11 isolates whose ISlO insertion was deleted by recombinational crossing-over, and in all cases the competitive fitness of the isolates was decreased. These results show that the ISIO-generated insertion increases fitness in chemostat cultures. We named the insertionjt::ISIO and suggest that transposable elements may speed the rate of evolution by promoting nonhomologous recombination between preexisting variations within a genome and thereby generating adaptive variation.