Sensory inputs to Drosophila sequential grooming

Animals integrate information from different sensory modalities, body parts, and time points to inform behavioral choice, but the relevant sensory comparisons and the underlying neural circuits are still largely unknown. I use the grooming behavior of Drosophila melanogaster as a model to investigate the sensory comparisons that govern a motor sequence. Flies perform grooming movements spontaneously, but when covered with dust, they clean their bodies following an anterior-to-posterior sequence. In the first part of this dissertation (chapter 2), I investigated the functions of different sensory modalities in grooming. I found multiple types of mechanosensory neurons can induce Drosophila grooming; other sensory modalities are not required. The grooming behaviors induced by different sensory organs are very different. Only activation of bristle neurons distributed over the body results in an anterior-to-posterior grooming sequence, similar to dust-induced grooming. In the second part of this dissertation (chapter 3), I investigated how sensory inputs contribute to grooming sequence. Computational modeling predicts that higher sensory input strength to the head will cause anterior grooming to occur first. I tested this prediction using an optogenetic competition assay where two targeted light beams independently activate mechanosensory bristle neurons on different body parts. I found that the initial choice of grooming movement is determined by the ratio of sensory inputs to different body parts. In dust-covered flies, sensory inputs change as a result of successful cleaning movements. Simulations from our model suggest that this change results in sequence progression. One possibility is that flies perform frequent comparisons between anterior and posterior sensory inputs, and the changing ratios drive different behavior choices. Alternatively, flies may track the temporal change in sensory input to a given body part to measure cleaning effectiveness. The first hypothesis is supported by our optogenetic competition experiments: iterative spatial comparisons of sensory inputs between body parts is essential for organizing grooming movements in sequence. In the last part of this dissertation (chapter 4), I investigated the neural circuit that processes sensory inputs from wing campaniform sensilla. Secondary interneurons are found in both brain and ventral nervous cord. Through an anatomy-guided behavior screen, I identified a group of interneurons which can induce wing grooming ipsilaterally