Chalmers Conferences, 9th European Conference on Mathematical and Theoretical Biology

Repair and not segregation of damage is the optimal unicellular ageing strategy
Robert J Clegg, Rosemary J Dyson, Jan-Ulrich Kreft

Last modified: 2014-04-01


How ageing, being unfavourable for the individual, can evolve is an unsolved problem of biology. Evidence for ageing in unicellular organisms is far from conclusive: some studies found a limited degree of ageing even in symmetrically dividing unicellular organisms, such as Escherichia coli; others did not find ageing in the same, or in different, unicellular organisms, or only under stress. Mathematical models suggested that segregation of non-genetic damage, i.e. ageing, would increase fitness. However, these models failed to consider repair as an alternative strategy or did not properly account for the costs and benefits of repair. We used a new and improved individual-based model to rigorously examine the effect of a range of ageing strategies on fitness in various environments.

Repair of damage emerges as the best strategy despite its fitness costs, since it immediately increases growth rate. There is an optimal investment in repair that outperforms damage segregation in well-mixed, lasting, and benign environments over a wide range of parameter values. Damage segregation becomes beneficial, and only in combination with repair, at high rates of damage accumulation if the damage is toxic and efficiency of repair low. Lineages of populations exhibiting small degrees of segregation reach steady states in our model, as in experiments, but we find that the difference in population fitness between little and no segregation is minute.

In contrast to previous models, our model predicts that unicellular organisms should have active mechanisms to repair damage. Indeed, as predicted, all organisms have evolved active mechanisms of repair whilst ageing in unicellular organisms is absent or accidental under benign conditions. Only microorganisms inhabiting short-lived environments, such as the budding yeast Saccharomyces cerevisiae and the bacterium Caulobacter crescentus, strongly favour early reproduction rather than longevity.

Ageing confers no fitness advantage for unicellular organisms in lasting environments under benign conditions, since repair of non-genetic damage is better than damage segregation.


ageing; microbiology; evolution