Experimental Evolution of Multicellular Complexity in Saccharomyces cerevisiae

The origin and evolution of multicellularity was directly investigated using experimental evolution. Using settling selection, multicellularity evolved quickly and repeatedly from a common unicellular ancestor, Baker's yeast. The transition occurred by persistent adhesion of daughter cells following cell replication. The resulting multicellular individuals had a morphology reminiscent of snowflakes, with many characteristics of extant multicellular species, including cell cell attachment, a single-cell bottleneck, and juvenile and adult life history stages. Cellular division of labor by apoptosis evolved in large snowflake clusters, ameliorating the effects of a trade-off between snowflake settling and growth rate. Continued settling selection led to additional adaptation, such as a more hydrodynamic cluster shape. The majority of the developmental changes that evolved after the transition to multicellularity were contingent on this transition and even on the specific mode of cluster formation. The origin and subsequent evolution of multicellular complexity in snowflake yeast can be directly attributed to natural selection.