CONNECTOMIC AND TRANSCRIPTOMIC INVESTIGATION OF THE OLFACTORY SYSTEM IN A MOLLUSC SUGGESTS A THIRD EVOLUTION OF GLOMERULAR ORGANIZATION

The olfactory systems of insects and vertebrates independently evolved a glomerular organization in which the axons of olfactory receptor neurons (ORNs) expressing the same odorant receptor (OR) converge onto the dendrites of the same projection neurons (PNs) in glomerular structures. Here we examined the olfactory system of the gastropod mollusc, Berghia stephanieae and found that it also exhibits the hallmarks of glomerular organization. In Berghia, ORNs are located in the rhinophores, dorsally projecting tentacles (Fig. A). We created a database of molluscan genes and probed it for olfactory-related gene sequences. G protein-coupled receptors (GPCRs) have been hypothesized to serve as ORNs in molluscs. We identified potential G proteins that they would couple with, finding that Golf (the homolog of the vertebration olfactory-related Gɑ subunit) is highly expressed in some regions of the rhinophore and Gq is expressed by cells in other regions. At the base of each rhinophore is a rhinophore ganglion (rhg). Neurobiotin fills of sensory neurons show that some terminate in the rhg (Fig. B). Reconstructing neurons and axons-of-passage in the rhg from segmented images in a volume electron microscopy (vEM) dataset, revealed afferent axons from distal rhinophore nerves converging on the dendrites of PNs, which each sent an axon proximally to the cerebral ganglion (Fig. C). Thus, the rhinophore has neurons, which display molecular signatures of ORNs and the rhg contains a glomerular circuit motif. Therefore, molluscs may represent a third example of convergent evolution of olfactory glomeruli.