Selective factors in the evolution of multi-cellularity

The evolution of multicellular animals from a unicellular ancestor represents a pivotal transition in life's history, and one of its greatest unsolved mysteries. Choanoflagellates (bacteria-eating protozoa that swim by undulating a flagellum) are the closest living relatives of animals. We have been using Salpingoeca rosetta, a unicellular choanoflagellate that can be induced to form multicellular colonies, as a model organism to study the evolution of multicellularity. For multicellularity to evolve, there must have been a selective advantage to being colonial, but since both unicellular and colonial choanoflagellates still exist today, there may be different environmental conditions under which single-celled or multicellular forms perform better. One important aspect of performance likely to affect choanoflagellate survival and reproduction is foraging (finding and capturing bacterial prey). Aquatic habitats can differ in the richness and patchiness of bacteria. We have been using S. rosetta to explore the functional consequences of being multicellular vs. unicellular, focusing on the physical mechanisms that determine foraging performance, which depends on (i) ability to find and retain position within patches of prey bacteria, and (ii) transport of prey in the flagella-produced water currents to the capture surfaces of the choanoflagellate. Protozoans, both unicellular and colonial, play an important role in aquatic food webs, so understanding the consequences of being uni- vs. multicellular to foraging performance in different habitats also has ecological significance. We are also studying how unicellular versus colonial choanoflagellates interact with different types of protozoan predators like those they would have encountered before muticellular animals evolved.

Selected references on this topic

  • Roper, M., M. J. Dayel, R. E. Pepper, N. King, and M. A. R. Koehl, (2013) Cooperatively generated stresslet flows supply fresh fluid to multicellular choanoflagellate colonies. Physical Review Letters 110: 228104 DOI: 10.1103/PhysRevLett.110.228104.

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