Lab-evolved multicellular organisms exhibit long-range flows that overcome nutrient diffusion limits

Large organisms face a challenge in transporting nutrients from outside to inside and waste from inside to outside. Early multicellular organisms were likely undifferentiated groups of cells and therefore would not have had specialized circulatory systems. Diffusion thus limits how far and fast nutrients can travel into these clusters of cells, setting a microscopic upper bound on size. Using a model organism for multicellularity, snowflake yeast, which has undergone ~5,000 generations of selection for larger size, we have observed that evolved clonal yeast clusters defy diffusion limits and grow exponentially to millimeter sizes. We present a possible mechanism for this: live clusters exhibit metabolically-driven flows when cultured in liquid media. Using micron-sized fluorescent beads, we have observed that fluid flows at 5-10 um/s into the bottom of the cluster and out through the top. We hypothesize that these flows are created due to variation in fluid density caused by gradients of nutrients or waste products. These results suggest that early multicellular organisms could have relied on physical processes alone — before the development of circulatory systems — in order to achieve large sizes in spite of diffusion limits.