Constraints on the G1/S transition pathway may favor selection of multicellularity as a passenger phenotype

Multicellularity has emerged in the three branches of the tree of life. The formation of simple multicellular entities can either result from cells aggregating or staying together after mitosis. However, it is not yet fully understood how, once formed, these simple multicellular entities could be maintained or even selected for. Here, using the ace2 yeast snowflake model of simple multicellularity, we aimed at identifying genetic conditions favoring its maintenance. Growth-competition experiments revealed that, while the ace2 mutation by itself does not provide any fitness advantage or disadvantage, the ace2 snowflakes were strongly selected when combined with conditions affecting regulators of the G1/S transition of the cell cycle, such as Cln3 or Whi5. We show that this selection results from a faster exit from quiescence of the ace2 snowflake cells. Importantly, this advantage is not dependent on the multicellular phenotype, but rather on the ace2 genotype itself. We found that the ace2 selective advantage in the cln3 background fully depends on the KSS1 gene, a target of the Ace2 transcription factor. Finally, we show that phenotypes observed for ace2 mutants are phenocopied by the AMN1^368D allelic form found in 'non-laboratory' yeast strains, hence adding physiological relevance to these observations. Altogether, our results support the hypothesis that simple multicellularity could, in some cases, persist, not because it provides a direct selective advantage due to multicellularity itself, but rather as a 'passenger' phenotype that is maintained alongside other selected traits.