Recent studies have shown that even in the absence of extrinsic stress, the morphologically symmetrically dividing model bacteria Escherichia coli do not generate offspring with equal reproductive fitness. Instead, daughter cells exhibit asymmetric division times that converge to two distinct growth states. This represents a limited senescence/rejuvenation process derived from asymmetric division that is stable for hundreds of generations. It remains unclear why the bacteria do not continue the senescence beyond this asymptote. Although there are inherent fitness benefits for heterogeneity in population growth rates, the two growth equilibria are surprisingly similar, differing by a few percent. In this work we derive an explicit model for the growth of a bacterial population with two growth equilibria, based on a generalized Fibonacci recurrence, in order to quantify the fitness benefit of a limited senescence process and examine costs associated with asymmetry that could generate the observed behavior. We find that a simple saturating effect of asymmetric partitioning of subcellular components is sufficient to explain why two distinct but similar growth states may be optimal while providing evolutionarily significant growth advantages.

最近的研究表明，即使在没有外部压力的情况下，形态上对称分裂的模型细菌大肠埃希菌不要产生具有相同生殖能力的后代。相反，子细胞表现出不对称的分裂时间，该分裂时间收敛到两个不同的生长状态。这代表了源于不对称分裂的有限的衰老/复兴过程，该过程可稳定数百代。尚不清楚为什么细菌不能在该渐近线之外继续衰老。尽管对于人口增长率的异质性具有固有的适应性好处，但两个增长均衡令人惊讶地相似，相差几个百分点。在这项工作中，我们基于广义斐波纳契递归得出了具有两个生长平衡的细菌种群生长的显式模型，为了量化有限的衰老过程的适应性益处，并检查与可能产生观察到的行为的不对称性相关的成本。我们发现，亚细胞成分不对称分配的简单饱和效应足以解释为什么两个不同但相似的生长状态可能是最佳的，同时提供了进化上显着的生长优势。