Few cell types, many functions: The evolutionary origin of division of labor among animal cells
EMBL, Heidelberg, Germany
Sponges are the sister group to nearly all other animals, and lack a nervous system, musculature, and gut. However, genes encoding important neuronal proteins, including key synaptic proteins, have been found in sponge genomes. Using single-cell RNAseq, single-molecule FISH, and Focused Ion Beam SEM (FIB-SEM) we generate a comprehensive molecular and morphological characterization of cell types in Spongilla lacustris, a freshwater demosponge. We identify many specialized cell types bearing functional and regulatory signatures similar to those of other animals. This includes contractile epithelial cells, which we demonstrate experimentally are responsive to nitric oxide signaling, phagocytes involved in innate immunity, and digestive cells that express a nearly complete set of postsynaptic genes. Remarkably, we also find immune cells expressing presynaptic genes and show via FIB-SEM that they send long projections that directly contact and enwrap microvilli of 'postsynaptic' digestive cells. This reveals new evidence linking neuronal and immune function in sponges, and suggests a primordial neuro-immune system cleared intruders and controlled ciliary beating for feeding.