UC Berkeley

Animal multicellularity requires cells to divide while remaining physically and developmentally integrated with their neighbors. Although this relationship is central to modern animal biology, how cell division contributed to the establishment of animal multicellularity is difficult to reconstruct because this transition occurred over 650 million years ago. This dissertation investigates the intersection of cell division and multicellularity in choanoflagellates, the closest living relatives of animals, and identifies septins as regulators of cell division and multicellular organization in the choanoflagellate Salpingoeca rosetta.Chapter 1 reviews the relationship between cell division and multicellular organization. I first discuss how cell division shapes animal tissues and how tissue environments constrain cytokinesis. I then consider how changes to cytokinesis and extracellular matrix regulation may have contributed to the emergence of clonal multicellularity. Finally, I introduce choanoflagellates, and S. rosetta in particular, as useful models for studying cell division and multicellular development in the context of animal origins.In Chapter 2, I investigate septin function in S. rosetta. Septins are cytoskeletal proteins that regulate cytokinesis in fungi and animals, but their functions in choanoflagellates were previously unknown. Using CRISPR/Cas9-mediated gene disruption, I found that multiple S. rosetta septins regulate cell size and rosette colony development. Further analysis of one septin, Sros_septA, shows that septin disruption causes late-stage cytokinesis failure and produces large multinucleated cells, phenotypes that become more pronounced following rosette induction. Endogenously tagged Sros_SeptA dynamically redistributes from the basal pole in interphase cells to the cleavage furrow and nascent intercellular bridge during division. Together, these findings suggest that septins regulate cytokinesis in S. rosetta and that the multicellular context influences septin function. Septins may therefore represent one mechanism that helped cytokinesis meet the physical and developmental demands of emerging multicellular organization during animal evolution.In Appendix 1, I present additional data on the localization of septins and actin in S. rosetta and discuss future directions for studying septin function at the interface of cell division and multicellularity in this model. In Appendix 2, I present preliminary work exploring the cellular and genetic basis of the phenotype in Solo, a mutant of S. rosetta defective in both chain and rosette colony development. Finally, in Appendix 3, I discuss preliminary work aimed at understanding the function of cadherins in choanoflagellates, genes whose adhesion and signaling functions in animals are critical for multicellular development.