Transposable elements as mutator genes in evolution

Abstract

Strains of the bacterium Escherichia coli habouring genes that increase mutation rates are known to have an evolutionary advantage in chemostat competition over otherwise isogeneic strains with lower mutation rates^1–3. This advantage is frequency-dependent, the mutator strain being favoured only above a starting ratio of ∼5×10⁻⁵, and it results from the fact that the necessary beneficial mutations cannot be generated in a mutator population below a certain size³. Here we consider the possibility that the mutagenic^4,5 properties of transposable elements confer an advantage in the same manner as mutator genes. A previous report has shown that the transposon Tn5 increases the fitness of E. coli in chemostats, although the reason for this effect has not been established⁶. Our results show that the transposon Tn10 also confers an advantage in chemostats. In addition, we find that (1) this advantage, like that associated with mutator genes, is frequency-dependent, (2) whenever the Tn10 strains win, a segment of Tn10, probably its IS10 sequences, has undergone transposition to a new site, (3) the new insertions converge into a site contained within a 3.2 kilobase (kb) PvuII fragment of the genome, and (4) no transpositions are detected when the Tn10 population lses. We conclude that Tn10 confers an advantage by increasing the mutation rate of the host bacterium.