Does symmetry preclude the evolution of senescence? A comment on Pen and Flatt 2021

Patterns of senescence across the tree of life remain poorly understood and a clearly important task is to identify the minimal conditions for senescence to occur at all. Senescence refers to changes in phenotype that cause an increase in mortality rate, or decrease in fertility rate, with age. Starting with Weismann in 1882, it has generally been argued that some type of asymmetry between parent and offspring is a prerequisite for old individuals to show declining performance [1–4]. The intuitive role of asymmetries should, however, be subject to mathematical scrutiny. Highly interestingly, recent work has highlighted results that counter the above intuition: Pen & Flatt (hereafter PF) recently reported that senescence can evolve in an organism that reproduces via symmetrical division, concluding that ‘[ … ] the evolution of senescence might, therefore, be inevitable [ … ]’ [5, p. 8].

Here we show that the ‘symmetric’ division of PF does not successfully remove all asymmetries between the two individuals that exist after reproduction. However, envisaging a fully symmetrical scenario is not straightforward either: assigning ‘age’ and the labels ‘parent’ and ‘offspring’ to individuals (or cells) presents non-trivial challenges. As a whole, this highlights that seeking for minimal conditions under which senescence can occur is a difficult task as it operates in a definitional minefield.