Is cell size a spandrel?

Abstract

All organisms control the size of their cells. We focus here on the question of size regulation in bacteria, and suggest that the quantitative laws governing cell size and its dependence on growth rate may arise as byproducts of a regulatory mechanism which evolved to support multiple DNA replication forks. In particular, we show that the increase of bacterial cell size during Lenski's long-term evolution experiments is a natural outcome of this proposal. This suggests that, in the context of evolution, cell size may be a 'spandrel'.

Keywords: B. subtilis; E. coli; bacteria; cell biology; cell cycle; cell size; evolution; infectious disease; microbiology; point of view.

Figure 1.. Phenomenological models for cell size control in bacteria.

This graph shows how the size at division, $f\left(v_b\right)$, depends on the size at birth (x-axis) in three different models. The parameter $\alpha$ is related to the slope of the function, and can continuously interpolate across different models. Its value can be determined from single-cell level correlations (Amir, 2014; see Marantan and Amir, 2016 for a recent generalization of this phase diagram). As $\alpha$ increases from 0 to 1, the correlations between mother and daughter cell sizes become weaker, yet the size distribution becomes narrower. The prevailing model for a critical size at initiation is effectively a ‘sizer’, and is inconsistent with recent experimental data supporting an ‘adder’. DOI: http://dx.doi.org/10.7554/eLife.22186.002

Figure 2.. Bacterial evolution and growth regulation.

E. coli cells evolving in a culture tube show both increasing fitness over time as well as increasing size. This can be naturally explained by a simple, specific regulation strategy, consistent with additional experiments. Figure adapted from Lenski and Travisano, 1994, with permission. DOI: http://dx.doi.org/10.7554/eLife.22186.003