Aging and Death in an Organism That Reproduces by Morphologically Symmetric Division

Abstract

In macroscopic organisms, aging is often obvious; in single-celled organisms, where there is the greatest potential to identify the molecular mechanisms involved, identifying and quantifying aging is harder. The primary results in this area have come from organisms that share the traits of a visibly asymmetric division and an identifiable juvenile phase. As reproductive aging must require a differential distribution of aged and young components between parent and offspring, it has been postulated that organisms without these traits do not age, thus exhibiting functional immortality. Through automated time-lapse microscopy, we followed repeated cycles of reproduction by individual cells of the model organism Escherichia coli, which reproduces without a juvenile phase and with an apparently symmetric division. We show that the cell that inherits the old pole exhibits a diminished growth rate, decreased offspring production, and an increased incidence of death. We conclude that the two supposedly identical cells produced during cell division are functionally asymmetric; the old pole cell should be considered an aging parent repeatedly producing rejuvenated offspring. These results suggest that no life strategy is immune to the effects of aging, and therefore immortality may be either too costly or mechanistically impossible in natural organisms.

Statistical study of asymmetry in cell lineage data

“…We applied our procedure to the Escherichia coli data of Stewart et al (2005). The biological issue addressed is aging in single cell organisms.…”

“…More recently, studies of aging in single cell organisms by Stewart et al (2005) suggested that cell division may not be symmetric. An asymmetric BAR model was therefore proposed by Guyon (2007), where the two sets of parameters corresponding to sister cells are allowed to be different.…”

“…For instance, Stewart et al (2005) filmed 94 colonies of Escherichia coli cells dividing between four and nine times. We therefore propose a new rigorous approach to take into account all the available information.…”

“…We study the consistency and asymptotic normality of our estimators and derive asymptotic confidence intervals as well as Wald's type tests to investigate the asymmetry of the data for both the BAR and GW processes. Our results are applied to the Escherichia coli data of Stewart et al (2005). We also provide an empirical study of the convergence rate of our estimators and of the power of the symmetry tests on simulated data.…”

“…In Section 4 we give our estimators and state their asymptotic properties. In Section 5, we propose a new investigation of Stewart et al (2005) data. In Section 6 we give simulation results.…”