Abstract
The evolution of multicellular life from single-celled ancestors is one of the most radical shifts in the history of life on earth, and sets the stage for evolution of more complex life forms. Despite the significance of this transition, we know little about the process by which cells first assemble groups and form multicellular organisms. We study this problem experimentally; a single mutation in the ACE2 gene of Baker’s yeast S. cerevisiae prevents mother and daughter cells from separating after cellular division. These yeast clusters, called ‘snowflake’ yeast, comprise a few hundred cells and grow to a maximum diameter of 200 microns. To evolve larger multicellular size, snowflake yeast clusters must mitigate forces strong enough to fracture cell-cell bonds. After a year of artificial selection for larger multicellular size, five populations of snowflake yeast surprisingly evolved to grow to a maximum diameter of 1 mm. In this work we investigate how nascent multicellular clusters evolve to overcome substantial mechanical constraints and dramatically increase their size.