Musings on senescence
The article in this Wednesday’s JC was Ageing and death in an organism that reproduces by morphologically symmetric division. by Stewart EJ, Madden R, Paul G and Taddei F. (PLoS Biol. 2005 Feb;3(2):e45. Epub 2005 Feb 01.), which deals with ageing in E. coli.
Single-celled organisms that reproduce by symmetric division have been held as functionally immortal. If the division is fully symmetrical, there is no way to distinguish between the mother cell and the daughter cell; in fact the distinction becomes meaningless since they are inseparable. In this case each cell division produces two infant cells, which leads to the conclusion that there is no ageing in single-celled organisms that reproduce by symmetric division.
The authors of the article propose that even morphologically symmetric division is not fully symmetric, e.g. not all of the cellular components are able to diffuse rapidly enough from the poles. The effect of this is that the concentration of these components will be different at the poles and in the center of the cell. Since the division occurs down the middle of the cell, this means that each of the new cells has one of these old poles and one new pole that is generated in the division. When either of these daughter cells divide, one of the granddaughter cells will have the two-generations old pole from the grandmother cell and one new pole, while the other will have the one-generation old pole from the mother cell and a new pole. This means that one of the granddaughter cells will be “older” than the other, even though they were “born” at the same time. In the article the authors report that they found that the cells that inherit the old pole “exhibit diminished growth rate, decreased offspring production and an increased incidence of death”. These results indicate that there is in fact functional ageing even in this kind of organism.
Why is this interesting? The question to ask about ageing can be posed as: “Why is there ageing?” or: “Why does ageing remain in the population when an individual who did not suffer from ageing, and could thus reproduce for a much longer period, would have an obvious evolutionary advantage?”. It would seem that not being subject to ageing would be a very successful strategy for an individual and also that even one single individual who gained a significantly longer reproductive period would skew the population quickly.
The results of the article suggest that there are no organisms that don’t age and the question why ageing has not been eliminated remains. One possible answer lies in a cost/benefit trade-off. Ageing sets in because the repair systems fail to keep up with the constant damage and degradation that our cells are subjected to. Repairs come with an energy cost that must be balanced against other costs such as those for reproduction and growth. This means that immortality, which would require a perfect repair-system, would come at a very high cost. Such a system would leave less energy to be expended in reproduction and growth, meaning that the individual would be less reproductive.
If we make the assumption that a perfect repair-system is possible and disregard any external threats, it seems that, given the enormous evolutionary advantage an immortal individual would have, a perfect repair-system would come at such a high cost that no energy would be left for growth and reproduction. IRL, however, every individual runs the risk of having their reproductive period (or life) foreshortened by violence or an accident. This makes it more costly to have low reproductivity, since it lessens the chance of successfully reproducing, and lowers the threshold at which the cost of a perfect repair-system is outweighed by the evolutionary advantage.
In conclusion: all organisms suffer from ageing, even those that reproduce by morphologically symmetric division. This means that it is likely that either it is not possible to have a perfect repair-system or that the cost of such a system would be large enough to outweigh the evolutionary benefit of immortality.