Regulation of Form in Multicellular Choanoflagellates and the Evolutionary Cell Biology of Morphogenesis

Proper morphogenesis requires the regulated interplay between cellular behaviors and physical constraints. Studies of physiology and morphogenesis in protist and animal systems with respect to this interplay have led to important mechanistic insights. Comparatively little work, however, has studied how morphogenetic processes evolve in the context of physical constraints. Choanoflagellates, the closest living relatives of animals, can form multicellular colonies of various shapes and sizes. This diversity and the simplicity of multicellular forms in conjunction with their important phylogenetic position makes choanoflagellates an ideal system for studying the evolution of morphogenesis. Because most work has focused on genetics and genomics, little is known about the cellular and biophysical mechanisms underlying the regulation of multicellular form in choanoflagellatesHere, I quantify the biophysical processes underlying the morphogenesis of rosette colonies in the choanoflagellate Salpingoeca rosetta. I find that rosettes reproducibly transition from an early stage of 2D growth to a later stage of 3D growth, despite the underlying stochasticity of the cell lineages. I postulate that the extracellular matrix (ECM) exerts a physical constraint on the packing of proliferating cells, thereby sculpting rosette morphogenesis. Perturbative experiments coupled with biophysical simulations demonstrate the fundamental importance of a basally-secreted ECM for rosette morphogenesis. In addition, this yields a morphospace for the shapes of multicellular colonies, consistent with observations across a range of choanoflagellates. Overall, this biophysical perspective on rosette development complements previous genetic perspectives and thus helps illuminate the interplay between cell biology and physics in regulating morphogenesis.I also present the previously undescribed species Choanoeca flexa, a splash pool choanoflagellate that forms cup-shaped colonies. The colonies rapidly invert their curvature in response to changing light levels, which they detect through a rhodopsin-cGMP pathway. Inversion is mediated by cell shape changes requiring actomyosin-mediated apical contractility and allows alternation between feeding and swimming behavior. C. flexa thus rapidly converts sensory inputs directly into multicellular contractions. In this respect, it may inform reconstructions of hypothesized animal ancestors that existed before the evolution of specialized sensory and contractile cells