Chemoreceptor Co-Expression inDrosophilaOlfactory Neurons

AbstractDrosophila melanogasterolfactory neurons have long been thought to express only one chemosensory receptor gene family. There are two main olfactory receptor gene families inDrosophila, the Odorant Receptors (ORs) and the Ionotropic Receptors (IRs). The dozens of odorant binding receptors in each family require at least one co-receptor gene in order to function:Orcofor ORs, andIr25a,Ir8a, andIr76bfor IRs. Using a new genetic knock-in strategy, we targeted the four co-receptors representing the main chemosensory families inDrosophila(Orco, Ir8a, Ir76b, Ir25a). Co-receptor knock-in expression patterns were verified as accurate representations of endogenous expression. We find extensive overlap in expression among the different co-receptors. As defined by innervation into antennal lobe glomeruli,Ir25ais broadly expressed in 88% of all olfactory sensory neuron classes and is co-expressed in 82% of Orco+ neuron classes, including all neuron classes in the maxillary palp.Orco,Ir8a, andIr76bexpression patterns are also more expansive than previously assumed. Single sensillum recordings from Orco-expressingIr25amutant antennal and palpal neurons identify changes in olfactory responses. These results suggest co-expression of chemosensory receptors is common in olfactory neurons. Together, our data present the first comprehensive map of chemosensory co-receptor expression and reveal their unexpected widespread co-expression in the fly olfactory system.