Environmentally regulated clonal-aggregative multicellularity in a choanoflagellate

Multicellularity evolved multiple times independently during eukaryotic diversification ^1–4 . Two distinct mechanisms underpin multicellularity ⁵ : clonality (serial cell division without sister-cell separation) and aggregation (whereby independent cells assemble into a multicellular entity). Clonal and aggregative multicellularity are traditionally considered mutually exclusive ^1,6–9 , with rare exceptions ¹⁰ , and evolutionary hypotheses have addressed why multicellularity might diverge toward one or the other extreme ^3,4 . Both animals and their sister group, the choanoflagellates, are currently only known to acquire multicellularity clonally ^4,11–13 . Here, we show that the choanoflagellate Choanoeca flexa ¹⁴ forms motile and contractile cell monolayers (or “sheets”) through multiple mechanisms: C. flexa sheets can form purely clonally, purely aggregatively, or by a combination of both processes. We characterise the life history of C. flexa in its natural environment – ephemeral splash pools on the island of Curaçao – and show that C. flexa undergoes reversible transitions between unicellularity and multicellularity during cycles of evaporation and refilling. Different splash pools house genetically distinct strains of C. flexa, between which aggregation is constrained by kin recognition ^15–18 . We show that clonal-aggregative multicellularity serves as a versatile strategy for the robust re-establishment of multicellularity in this variable and fast-fluctuating environment. Our findings challenge former generalisations about choanoflagellates and expand the option space of choanozoan multicellularity.