The Population Biology of Bacterial Transposons: A Priori Conditions for Maintenance as Parasitic DNA

We have theoretically and quantitatively analyzed the population dynamics of transposable elements in populations of bacteria and plasmids. Computer simulations were used to describe populations of cells with and without plasmids and transposons. Plasmids could enter new cells during conjugation events, and transposons could enter new pieces of DNA (either a plasmid or the chromosome) by transposition. Because both processes could be reversed, either element could be lost from a cell. The model demonstrates that a conservative transposon cannot increase in frequency by transposition alone but that a replicating one can. Plasmid transfer is necessary as the vehicle by which transposons penetrate new cells. The rate at which a selectively neutral transposon increases in frequency is limited by transposition rate or plasmid turnover rate, whichever is smaller. A fitness cost greater than this maximum invasion rate results in the elimination of the transposon from the population. Since both transposition and plasmid turnover rates are generally extremely low in natural populations of bacteria, usually below 10⁻³ and often 10⁻⁵ to 10⁻⁷ per cell per hour, we argue that transposons would be ineffective as parasites of bacteria. We thus argue against the generality of the hypothesis that transposons are parasitic DNA in prokaryotes. It is more likely that most transposons are currently maintained in populations and invade new strains by augmenting the fitness of their hosts.