Rugged landscapes and evolutionary paths to variations in extreme size

The evolutionary transition to multicellularity transformed life on earth, allowing for the evolution of large, complex organisms. While multicellularity can be strongly advantageous, its earliest stages bring unique physical challenges, including the need to mitigate internal and external stresses. Previous work (Jacobeen et.al. Nature Physics 2018 and Jacobeen et.al. PRE 2018) used the model experimental system “snowflake yeast”, a baker’s yeast (S. cerevisiae) genetically modified to remain attached via uncut chitin bonds during mother-daughter budding. However, so far these studies have focused on unimpeded directional selection for large size. Here, we varied the selection protocol to probe the effects of a more complex and rugged fitness landscape on the evolutionary trajectory of nascent multicellular clusters by subjecting the population to external compression prior to selection for larger size. We find that despite this challenge, the maximum cluster size achieved over eight weeks of experimental evolution is unchanged by the presence of compression. This indicates that significant evolutionary changes are possible even under harsh environmental conditions, and that very different selection environments can yield similar phenotypic variation.