Slime Mould and the Transition to Multicellularity: The Role of the Macrocyst Stage

The transition from unicellular to multicellular organisms is one of the mysteries of evolutionary biology. Individual cells must give up their rights to reproduction and reproduce instead as part of a whole. I review and model the macrocyst stage in slime mould (Dictyostelium) evolution to investigate why an organism might have something to gain from joining a collective reproduction strategy. The macrocyst is a reproductive cartel where individual cells aggregate and form a large zygotic cell which then eats the other aggregating cells. The offspring all have the same genetic code. The model is a steady state genetic algorithm at an individual cellular level. An individual’s genetic code determines a threshold above which it will reproduce and a threshold below which it will join a macrocyst. I find that cycles in food availability can play an important role in an organism’s likelihood of joining the macrocyst. The results also demonstrate how the macrocyst may be an important precursor to other cooperative behaviours.