Sex vs. Strong Mutators

In large populations of asexually reproducing bacteria, novelty is introduced mainly by mutation. One can see that high rates of mutation might allow such a population to track environmental changes quickly. What is more, there are genes that act to raise mutation rates in the genes close to them: they are called strong mutators. Because the strong mutators remain close to the mutations that they help to generate, a rapidly evolving population of bacteria will contain, and perhaps will accumulate, strong mutator genes. Obviously, there will be a heavy load of harmful mutations within such a population, but that can be tolerated if environmental conditions keep on favoring mutants. Such a genetic structure, with a lot of strong mutators, is of course evolved under normal natural selection. But the effect of the genetic structure is to set up a situation in which many "species" of bacteria could arise. Because of what has been selected as a first-order effect, the net result could be extreme and rapid diversification on a larger scale.

Strong mutators have been identified that can increase mutations 50-fold compared with strains of bacteria that do not have them. So what would act to prune out strong mutators? There are at least three possibilities:

1. Bacteria might be well adapted to their environments, so "normal", weak mutation rates might be sufficient. Strong mutators would then select themselves out of the genome, taken out by the dominantly harmful mutations they have generated.
2. Bacteria might be more "generalist" (less specialized) than we think, so would not need the continuous "specialist" mutations that would track environmental changes quickly and precisely.
3. Strains of bacteria could gain genetic novelty in less costly ways than mutation, cutting the cost of harmful mutations while still generating the genetic diversity they need to track environmental fluctuations or other selective factors. Such alternative methods would be varieties of bacterial conjugation (genetic exchange) = sex.

Tenaillon et al. (2000) ran simulations to test this latter idea. As the rate of genetic exchange increased, the value of fixing a strong mutator decreased. Sexual exchange, of course, makes adaptation faster, because it increases the chances that two separate beneficial mutations can be combined quickly into one genome, without waiting for those two mutations to be evolved asexually in sequence, one after the other. The simulations suggest that recombination allows this to happen more often than even strong mutators can generate it.

Reference: Tenaillon, O., et al. 2000. Mutators and sex in bacteria: conflict between adaptive strategies. Proceedings of the National Academy of Sciences, preprint, published on-line before the journal issue, Fall 2000.

Page written by RC, November 25, 2000.