Evolution of irreversible differentiation under stage-dependent cell differentiation.

The specialization of cells is a hallmark of complex multicellularity. Cell differentiation enables the emergence of specialized cell types that carry out separate functions previously executed by a multifunctional ancestor cell. One view about the origin of cell differentiation is that it first occurred randomly in genetically identical cells exposed to the same life history environment. Under these conditions, differentiation trajectories producing more offspring could be favored by natural selection; yet, how dynamic variation in differentiation probabilities can affect the evolution of differentiation patterns is unclear. We develop a theoretical model to investigate the effect of dynamic-stage-dependent-cell differentiation on the evolution of optimal differentiation patterns. Concretely, we model trajectories in which cells can randomly differentiate into germ or soma cell types at each cell division. After comparing many of these trajectories, we found that irreversible differentiation, where cells gradually lose their ability to produce the other cell type, is more favored in small organisms under dynamic than under constant (stage-independent) cell differentiation. Furthermore, we found that the irreversible differentiation of germ cells, where germ cells gradually lose their ability to produce soma cells, is prominent among irreversible patterns. Only large variations in the differentiation probabilities prohibit irreversible trajectories from being the optimal pattern.