Mapping courtship song representations throughout the central brain of Drosophila melanogaster

Abstract

Many animals use sound for communication, from insects to mammals, but we do not yet understand how the brain encodes auditory information in order to extract the information salient for behavioral decisions (e.g. to flee when hearing an alarm call or to approach when hearing a courtship song). One of the main challenges, in any system, has been mapping sensory information past the periphery, and identifying the patterns of neural activity throughout the entire brain involved in representing sensory information (of any modality) - in addition, we lack tools to compare this activity across animals. Here I present new methods for monitoring auditory activity throughout the entire central brain of the vinegar fly, Drosophila melanogaster, at cellular resolution, along with methods for registering activity across trials and brains for direct comparisons. I utilize these new methods to map and characterize auditory representations past the periphery, and investigate how tuning for courtship song-features arises, in addition to examining the reliability of responses across stimulus presentations and individuals. The fly is an excellent model system for addressing questions related to auditory coding because of its compact nervous system, extensive genetic tools, and robust acoustic communication-based behaviors. During courtship, males chase females while producing a dynamic courtship song via unilateral wing vibration. Females extract information from these songs, which informs their mating decisions, while males eavesdrop on the songs of potential competitors. D. melanogaster song comprises only three modes, two types of brief sound impulses termed ‘pulse song’ and one hum-like ‘sine song’. Using a brain-wide approach to study auditory coding in Drosophila, I record from 18,765 stimulus-modulated regions-of-interest across 31 male and female flies. I uncover that auditory activity is diverse and spatially widespread, finding dense auditory activity in several brain regions not previously known to be auditory, including several neuropils of the olfactory system. I find that stimulus-selectivity and temporal properties of auditory responses differ between neuropils, with activity profiles becoming more diverse from primary auditory areas to more downstream and multimodal neuropils. I also demonstrate that auditory responses are more stereotyped across trials and animals in early auditory areas (neuropils that are one to two synapses downstream of the auditory periphery). I find that only one neuropil, the gnathal ganglia, shows strong dimorphic responses between males and females. Finally, I probe stimulus selectivity in more detail in four neuropils, and find that auditory activity can be divided into three distinct classes - neurons preferring sustained pure tones, pulses, or mixtures of the two. In addition, using a novel class of stimuli, I find that several neurons belonging to these classes are sensitive to the envelope modulation of the stimulus, providing new information on how sine versus pulse selectivity emerges in the auditory pathway. These results highlight the power of studying brain-wide sensory-driven activity to provide a systems-level understanding of how sensory stimuli are encoded.