Eukaryote Aggregative Multicellularity: Phylogenetic Distribution and a Case Study of Its Proximate and Ultimate Cause in Dictyostelia

Aggregative multicellularity, resulting in fruiting body (sorocarp) formation is a response to starvation stress that evolved many times independently across eukaryotes. This chapter summarizes the life cycles and phylogenetic distribution of sorocarpic eukaryotes and examines the largest group of sorocarpic organisms, the Dictyostelia, in more detail. Regulation of the multicellular life cycle of the model Dictyostelium discoideum is dominated by cyclic AMP (cAMP), which acts intracellularly to induce spore and stalk cell differentiation and extracellularly to coordinate aggregation and fruiting body morphogenesis. This chapter highlights how these functions of cAMP gradually evolved from an original role as intermediate for starvation-induced encystment in a unicellular ancestor. It further reports how adaptation to a colder climate has likely driven the evolution of multicellular sporulation in Dicytostelia.