The Solomon H. Snyder Department of Neuroscience, The Center for Sensory Biology, Johns Hopkins University School of Medicine Baltimore, Maryland
The vinegar fly Drosophila melanogaster uses its sense of smell for many ethologically relevant behaviors. Flies primarily use one of two classes of chemoreceptors to detect odors: Odorant Receptors (ORs) and Ionotropic Receptors (IRs). These chemoreceptors require co-receptors for ion channel formation and proper dendritic trafficking. The co-receptor for the ORs is Orco (Odorant Receptor COreceptor), and the co-receptors for the IRs are Ir8a, Ir25a, and Ir76b. Fly olfactory neurons are thought to express only one type of chemoreceptor class per neuron. Thus, these different olfactory modalities were previously thought to be anatomically and functionally distinct and mutually exclusive. To evaluate this dogma in greater detail, we extended the HACK genetic technique (Homology Assisted CRISPR Knock-in) developed in our lab to generate targeted knock-ins of all four Drosophila olfactory co-receptor genes, and discovered that the IRs are much more broadly expressed than previously reported. The IR modalities overlap extensively with each other, as well as with Orco+ olfactory neurons. We performed electrophysiological recordings from Orco+ neurons in one of the fly’s two olfactory organs, the maxillary palps, and found olfactory responses influenced by Ir25a function in these neurons. Finally, we used RNAseq to identify potential 'tuning' IRs in the palps. These results demonstrate that fly olfactory neurons have the potential to be polymodal odor detectors, and suggest the need for a re-evaluation of odor coding in Drosophila and other insects.