Dynamics, stability, and robustness of minimal-change trajectories to increased multicellular size

The evolution of large organismal size is fundamentally important for multicellularity, creating new ecological niches and opportunities for the evolution of increased biological complexity. Yet little is known about how large size evolves, particularly in nascent multicellular organisms that lack genetically-regulated multicellular development. Here we examine how novel biophysical drivers of macroscopic multicellularity arise, including both the minimal requirements which drive novel biophysical adaptation and the dynamics of their emergence, using a combination of experiments and simulations. In a long-term evolutionary experiment, we observed multiple independent lines of multicellular snowflake yeast evolve macroscopic size rapidly, becoming 20,000 times larger and 10,000 times more physically tough over a span of 600 days. They accomplished this through sustained biophysical adaptation, evolving a novel trait, branch entanglement, so that groups of cells stay together after bond fracture. We developed simulations to interrogate the minimal changes that can be made to cellular and growth morphologies to facilitate branch entanglement. We find that cellular elongation, a trait observed in all lines of multicellular yeast experimental evolution, provides a robust and easy mechanism to delay the onset of bond fracture such that branch entanglement is more likely. Our simulations and experiments show that the emergent properties of simple multicellular groups can drive sustained biophysical adaptation, an early step in the evolution of increasingly complex organisms. In future work, we will continue to investigate the effects of other physical changes that may facilitate branch entanglement, including increasing bond strength, decreasing cell stiffness, or increasing the budding angle for more severe angles between parent and child cells.