PROJECT SUMMARY/ABSTRACT The goal of this project is to determine the organizing principle underlying olfactory receptor neuron (ORN) grouping in the antennal sensory hairs of Drosophila. For most sensory systems, neurons are arranged in an ordered manner to form topographic representations of stimuli from the external world. Understanding the organizing principle of such maps is critical for deciphering the nature of salient features that are extracted from parallel inputs through lateral inhibition. However in olfaction, whether and how neurons are organized into a sensory map remains largely unclear. In both rodents and flies, it is reported that neighboring glomeruli do not necessarily respond to similar odorants. In the absence of a chemotopic map, what then is the organizing principle for the olfactory system? To address this complex question, the research outlined here leverages the powerful genetic toolkit and tractable olfactory system of Drosophila. The general hypothesis— that ORNs are organized in the periphery based on the hedonic value of their detected odors to form a valence map—will be tested via systematic behavioral assays with optogenetic or thermogenetic activation of select receptor neurons. The proposal’s first aim is to determine the generality of the observation that in the same sensory hair, compartmentalized ORNs mediate opposing behavior—with attraction elicited by activation of the large-spike ORN(s) and aversion by the small-spike neighbor(s). The remaining aims propose to examine this organizing principle in sensory hairs implicated in regulating female egg-laying (Aim 2) or male courtship behavior (Aim 3). Successful execution of the proposal will likely bridge a longstanding knowledge gap in sensory biology by revealing, for the first time, olfactory maps within a complex organism.