A Yeast Model for the Evolution of Multicellularity and Cellular Differentiation

Multicellularity and cellular differentiation have each evolved independently in over a dozen clades. Most extant organisms either exhibit both traits or possess neither one, though it is presumed that these traits evolve serially. Several hypotheses have been proposed to explain why the transitional forms (single‐celled, differentiating species, or vice versa) may be less fit or more evolutionarily unstable. To test these predictions, we have engineered strains of the budding yeast S. cerevisiae that can differentiate into two cell types and/or form clonal aggregates. (We model “germ‐soma” differentiation using an engineered gene excision system in which somatic cells irreversibly lose reproductive potential and induce expression of genes whose products benefit nearby germ cells.) By comparing these strains, we find that multicellularity (i) enhances the fitness benefits that somatic cells can provide to germ cells due to increased proximity and (ii) is protective against the invasion of a non‐differentiating mutant. By contrast, the costs of differentiation do not outweigh the benefits in our unicellular strains. These results buttress theoretical predictions that evolution through a multicellular, non‐differentiating intermediate is the most likely path to differentiating multicellular organisms, and that these two traits act synergistically to produce a more fit and stable phenotype.