A bottom-up model of the evolution of multicellular reproductive strategies

Renske Vroomans
University of Cambridge, UK
The evolution of multicellularity is one of the most ubiquitous transitions in individuality. Evolutionary experiments find that many prokaryotic and eukaryotic species can rapidly evolve multicellularity when selection favours group formation. Unicellular species that are closely related to animals or land plants often possess many of the signalling molecules, transcription factors and e.g cadherins that are important for multicellular development. All in all, this suggests that the transition to multicellularity likely involved a remarkably complex unicellular ancestor, with a large genetic toolkit, and an intricate life cycle which involved considerable changes in the behaviour of the cell. Then, at the transition to multicellularity, some of these behaviours were likely co-opted to organise the development of the nascent multicellular organism. From this starting point, we developed evolutionary cell-based models to show how the collective behaviour of complex cells drives the evolution of regulated multicellularity. We modelled a population of cells that can evolve the regulation of their behavioural state - either division or migration - and studied both a unicellular and a multicellular context. Cells compete for reproduction and for resources to survive in structured environments. We find that in different environments, different cellular strategies evolve, ranging from fully unicellular to exclusively multicellular. In a subset of environments we find that group-level reproductive strategies evolve, depending on the distribution of the food sources. I will discuss how these models allow us to study the transition in individuality as a process without a threshold, where both cells and groups remain accessible surfaces for evolutionary change.