Figure 5. Growth yield and growth rate of the selected and wild strains. No significant correlation was seen between growth yields and growth rates. (t = 20.157, p = 0.181). Colour codes same as figure 3. doi:10.1371/journal.pone.0014516.g005 before selection in response to high and low nutrient concentra- tions. It is important to note that despite this change the response to nutrient concentration remained the same demonstrating the robustness of this association. Prolonged selection in high nutrient concentrations induced commitment to asymmetric cell division in the cultures and selection in low nutrients caused commitment to symmetric cell division. This might reflect that there is a cost associated with phenotypic plasticity owing to which under a monotonous regime the plasticity was lost. A subtle difference between the two types of commitments was that commitment to asymmetric cell division was faster than commitment to symmetric cell division. At 1000 generations of selection all the three strains being selected under high nutrient concentration showed com- mitment to asymmetric division. The L lineages, on the other hand retained plasticity. After 2000 generations all the three H strains showed commitment to asymmetry but only two of the three L showed commitment to symmetric division. This may lead to a speculation that asymmetric division may be associated with a lower cost than symmetric division, although it may be too early to make such inferences. In an experimental set up such as the above, it is difficult to define a dead cell. We assume cells that stop dividing during the observation period as presumably dead and cells that divide slowly as old. The former is particularly difficult to validate. However if we accept this as a working definition, the proportion of old and dead cells was greater in the H selected lineages that were committed to asymmetric division. This observation is compatible with the model prediction [10] that symmetric division may delay or avoid aging. The results are compatible with the theoretical prediction [10] that asymmetric division and aging will evolve in nutrient rich environments and symmetric cell division and non-aging popula- tion may evolve in caloric restricted environments. Correlations with growth rates and growth yields We did not find a positive correlation between growth rates and index of asymmetry as the model had predicted. Growth rates in high current concentrations were greater in 6 out of 9 strains where as they did not differ substantially in the remaining 3. It is evident therefore that the low nutrient concentration was growth limiting. It is interesting to note that even in the strains where growth rates in h and l did not differ, the asymmetry index was Bacterial Division Symmetry significantly different. This is suggestive of at least partial independence of the mechanisms to regulate growth rates and cell division symmetry. Also contrary to our expectations H selection did not give rise to strains with higher growth rates than L selection. As opposed to growth rates, Growth yields correlated negatively with the index of asymmetry consistent with the model prediction but we did not find the expected negative correlation between growth rates and growth yields. It appears therefore that although the model prediction about cell division symmetry was well supported by the experiments, the predictions about growth rate and growth yields were not quite well supported. However, caution is needed in inferring from these result. In the design of the experiment, cell divisions had to be monitored on agar surface since liquid cultures do not allow monitoring of the same cell over a long time, and growth yields could be estimated only in liquid media where even distribution of nutrients throughout the medium can be ensured. This inevitable flaw in the experiment could have led to certain artefacts. For example the growth parameters in liquid and solid media may be sufficiently different to distort the correlations. The inability to get different growth rates in H and L lines may be owing to the selection regime which selected for higher growth rates in either of the nutrient concentrations. Growth rates increased after subculturing in high as well as low nutrient concentrations and it is likely that other mechanisms facilitating growth rate might have dominated the results masking the effect of cell division symmetry on growth rate. Symmetric cell division should be accompanied by the optimization of repair rates according to the Watve et al. [10] model. Our experiments could not test this prediction directly. However, in the model, increase in the growth yield accompany- ing symmetric cell division critically depended on optimization of repair or recycling rates. Therefore higher growth yield can be taken as an indirect evidence for higher repair rates accompanying symmetric cell division. Is the association more general? It is well known that caloric restriction leads to longevity in widely differing organisms including yeast, Caenorhabditis elegans, Drosophila, and mammals although it cannot be said to be universal [16,17]. The association between cell division symmetry and caloric restriction could be a more general phenomenon in unicellular organisms. Mutation in Sir2, a gene necessary for asymmetric segregation in yeast [8], resulted in a longer chronological life span under caloric restriction [18]. It appears that Sir2 activation, leading to asymmetric segregation, reduces the chronological life span in yeast [18,19] although its role in reproductive life span extension is debated [19,20]. Other indications such as higher growth yields in oligophilic bacteria [21], growth rate and growth yield trade off [14,15] suggest that the relationship between caloric restriction, cell division symmetry, high repair rates and high growth yield could be more generally true and responsible for longevity in widely differing organisms. Asymmetry of division may have comparable implications in unicellular and complex organisms. In unicellular organisms asymmetric division may have evolved to increase the growth rate of the population by continually generating young cells, where as in multicellular organisms asymmetric division seems to rejuvenate stem cells [22–25]. However, the consequences of symmetric division for aging may be different in unicellular and multicellular organisms. Caloric restriction induced symmetric division may arrest aging in unicellular organisms but this mechanism need not be relevant to aging in multicellular organisms. Instead enhanced repair rate in response to caloric restriction predicted by the model could be more relevant. PLoS ONE | www.plosone.org 5 January 2011 | Volume 6 | Issue 1 | e14516