Cell Death in Evolutionary Transitions in Individuality.

Programmed cell death (PCD) in cell groups and microbial communities affects population structures, nutrient recycling, and sociobiological interactions. A less explored area is the role played by PCD in the emergence of higher-level individuals. Here, we examine how cell death impacted evolutionary transitions in individuality (ETIs). The focus is on three specific ETIs - the emergence of the eukaryote cell, multicellularity, and social insects - and we review the theoretical and empirical evidence for the role of PCD in these three transitions. We find that PCD likely contributed to many of the processes involved in eukaryogenesis and the transition to multicellularity. PCD is important for the formation of cooperative groups and is a mechanism by which mutual dependencies between individuals evolve. PCD is also a conflict mediator and involved in division of labor in social groups and in the origin of new cell types. In multicellularity, PCD facilitates the transfer of fitness to the higher-level individual. In eusocial insects, PCD of the gonadal cells in workers is the basis for conflict mediation and the division of labor in the colony. In the three ETIs discussed here, PCD likely played an essential role, without which alternate mechanisms would have been necessary for these increases in complexity to occur.