Emergence of heritability of higher-level traits in a major transition

Increases in biological complexity and the origins of life’s hierarchical organization are described by the “major transitions” framework. A crucial component of this paradigm is that after the transition in complexity or organization, adaptation occurs primarily at the level of the new, higher-level unit. For collective-level adaptations to occur, though, collective-level traits—properties of the group, such as collective size—must be heritable. Using the ‘snowflake yeast’ model system of early multicellularity, we investigated the biophysical basis of emergent multicellular traits. We genetically engineered four yeast genotypes to have different cellular aspect ratios, which prior work has shown affects cellular packing and the size of the multicellular group. We then measured the heritability of both the cellular trait (aspect ratio) and emergent multicellular trait (group size at fracture) and compared them to our theoretical predictions. Extrapolating from these results, we show mathematically that, for a wide range of functions relating the group traits to cellular traits, the emergent group-level traits will have higher heritability than their corresponding cell-level traits.