Mixed clonal-aggregative multicellularity entrained by extreme salinity fluctuations in a close relative of animals

Multicellularity evolved multiple times independently during eukaryotic diversification1-4. Two distinct mechanisms underpin multicellularity5: 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 exclusive1,6-9, with rare exceptions10, and evolutionary hypotheses have addressed why multicellularity might diverge toward one or the other extreme3,4. Both animals and their sister group, the choanoflagellates, are currently only known to acquire multicellularity clonally4,11-13. Here, we show that the choanoflagellate Choanoeca flexa14 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 Curacao - 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 recognition15-18. 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.