The architecture of cell differentiation in choanoflagellates and sponge choanocytes

Collar cells are ancient animal cell types which are conserved across the animal kingdom and their closest relatives, the choanoflagellates. However, little is known about their ancestry, their subcellular architecture, or how they differentiate. The choanoflagellate Salpingoeca rosetta expresses genes necessary for animal multicellularity and development and can alternate between unicellular and multicellular states making it a powerful model to investigate the origin of animal multicellularity and mechanisms underlying cell differentiation. To compare the subcellular architecture of solitary collar cells in S. rosetta with that of multicellular 'rosettes' and collar cells in sponges, we reconstructed entire cells in 3D through transmission electron microscopy on serial ultrathin sections. Structural analysis of our 3D reconstructions revealed important differences between single and colonial choanoflagellate cells, with colonial cells exhibiting a more amoeboid morphology consistent with relatively high levels of macropinocytotic activity. Comparison of multiple reconstructed rosette colonies highlighted the variable nature of cell sizes, cell-cell contact networks and colony arrangement. Importantly, we uncovered the presence of elongated cells in some rosette colonies that likely represent a distinct and differentiated cell type. Intercellular bridges within choanoflagellate colonies displayed a variety of morphologies and connected some, but not all, neighbouring cells. Reconstruction of sponge choanocytes revealed both ultrastructural commonalities and differences in comparison to choanoflagellates. Choanocytes and colonial choanoflagellates are typified by high amoeboid cell activity. In both, the number of microvilli and volumetric proportion of the Golgi apparatus are comparable, whereas choanocytes devote less of their cell volume to the nucleus and mitochondria than choanoflagellates and more of their volume to food vacuoles. Together, our comparative reconstructions uncover the architecture of cell differentiation in choanoflagellates and sponge choanocytes and constitute an important step in reconstructing the cell biology of the last common ancestor of the animal kingdom