Abstract

Multicellularity evolved multiple times independently during eukaryotic diversification. Two distinct mechanisms underpin multicellularity: clonal development (serial cell division of a
single precursor cell) and aggregation (in which independent cells assemble into a multicellular entity). Clonal and aggregative development are traditionally considered to be mutually exclusive and to result from independent acquisitions of multicellularity. Here, we show that the choanoflagellate Choanoeca flexa, a close relative of animals that forms contractile monolayers of cells (or “sheet colonies”), develops by an unconventional intermediate mechanism that we name “clonal-aggregative multicellularity”. We find that C. flexa sheets can form through purely clonal processes, purely aggregative processes, or a combination of both, depending on experimental conditions. To assess the ecological relevance of these findings, we characterize the natural context of multicellular development in the native environment of C. flexa on the island of Curaçao. We show that the C. flexa life cycle is environmentally regulated by extreme salinity fluctuations in splash pools undergoing cycles of evaporation and refilling. Upon desiccation, C. flexa colonies dissociate into drought resistant quiescent cells, which resume activity and reform multicellular sheets after rehydration. We hypothesize that clonal-aggregative development reflects selection for fast transitions into and out of multicellularity in the ephemeral context of coastal splash pools. Our findings underscore the potential of the exploration of biodiversity for revealing new fundamental biological phenomena and expand the known option space for both multicellular development and for the origin of animal multicellularity.