Emergent structure enables lab-evolved multicellular organisms to overcome nutrient diffusion limits

Diffusion limitation is widely thought to be a fundamental constraint on multicellular size prior to the evolution of adaptations that overcome it, such as circulatory systems. Nutrients often cannot penetrate more than ~100µm into a group of unspecialized cells, meaning that cells on the inside of a large group will starve, and growth will be constrained. However, our model organism for early multicellularity, snowflake yeast, defies these uptake limits. Over ~5,000 generations of selection for large size, these clonal yeast clusters evolved to grow exponentially to millimeter sizes in both still and shaken media, growing far larger than previously-demonstrated uptake limits. In this project, we explore how the internal structure of these multicellular clusters facilitates fluid flow through them. As snowflake yeast evolved, the individual cells became much longer and the bonds between them became stronger, eventually creating an entangled, porous structure emerging solely from these cell-level characteristics. Our results suggest that early multicellular organisms could have relied on emergent structure alone — before the development of complex multicellular adaptations — in order to take advantage of flows to bring nutrients to the whole group.