Origin of Cellular Senescence in Ciliates

Mixing and sharing of genes is essential for population diversity, which contributes to the resilience and, ultimately, the survival of animal and plant communities. However, sharing of genes is antithetical to individual fitness; hence gene mixing is threatened with extinction if selection rewards selfish (clonal) reproduction. To address this dilemma, mechanisms have evolved to enforce a mandate for gene sharing. In most metazoa, the functions of sex and reproduction are tightly entwined, presumably for the purpose of compelling the sharing of genes. In protists, the functions of sex (conjugation) and reproduction (amitosis) are separate. The mandate for gene sharing is enforced, instead, by a form of cellular senescence. Ciliates can reproduce clonally a few hundred times before they senesce and die. Conjugation resets their biological clock, restarting the cycle of clonal reproduction. The question how reproduction in metazoa came to be linked to sex has been explored in the past, but a fully satisfying account in terms of evolutionary ecology is lacking. The question how senescence in protozoa came to be linked to sex has not been addressed, and is the topic of the current study. I present herein two numerical simulations (IBMs) for the evolution of cellular senescence in ciliates. The results shed light not only on the evolution of senescence in higher life forms, but on more general questions concerning the plausibility of group selection.