Abstract
Aging has been demonstrated in unicellular organisms and is presumably due to asymmetric distribution of damaged proteins and other components during cell division. Whether the asymmetry-induced aging is inevitable or an adaptive and adaptable response is debated. Although asymmetric division leads to aging and death of some cells, it increases the effective growth rate of the population as shown by theoretical and empirical studies. Mathematical models predict on the other hand, that if the cells divide symmetrically, cellular aging may be delayed or absent, growth rate will be reduced but growth yield will increase at optimum repair rates. Therefore in nutritionally dilute (oligotrophic) environments, where growth yield may be more critical for survival, symmetric division may get selected. These predictions have not been empirically tested so far. We report here that Escherichia coli grown in oligotrophic environments had greater morphological and functional symmetry in cell division. Both phenotypic plasticity and genetic selection appeared to shape cell division time asymmetry but plasticity was lost on prolonged selection. Lineages selected on high nutrient concentration showed greater frequency of presumably old or dead cells. Further, there was a negative correlation between cell division time asymmetry and growth yield but there was no significant correlation between asymmetry and growth rate. The results suggest that cellular aging driven by asymmetric division may not be hardwired but shows substantial plasticity as well as evolvability in response to the nutritional environment.
Highlights
Aging is known to occur in bacteria and yeast that divide with morphological asymmetry [1,2,3]
Index of cell division time asymmetry For quantifying functional symmetry in cell division we defined an index of cell division time asymmetry based on the assumption that if the cell division was asymmetric, the daughter cell receiving older components will take longer to complete the cell division [4,5,7]
In all the 12 experiments the index of asymmetry was greater under high nutrient concentration as compared to low (Figure 1A–C) and 10
Summary
Aging is known to occur in bacteria and yeast that divide with morphological asymmetry [1,2,3]. The two cells after division can be viewed as a parent cell and a daughter cell rather than as two sister cells This gives rise to a population dynamics that is similar to multicellular organisms that show age structured populations. Based on a Leslie Matrix model Watve et al [10] simulated symmetric and asymmetric division and its effects on population growth. Their model assumed that the efficiency of cellular components declined with age and growth rate of a cell was a function of the relative proportions of new and old components. Based on the simulation results Watve et al [10]
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