Single-Cell Analysis of Somatosensory Neuron Projection Patterns in the Nerve Cord and Brain of the Fruit Fly
1 Institute of Zoology, University of Cologne, Germany
2 Institute of Molecular and Cellular Biosciences, The University of Tokyo, Japan
Although some somatosensory neurons terminate only in the nerve cord, certain subsets of sensory neurons project not only to the nerve cord but also to the brain in both vertebrates and insects. Although the nerve cord is a key center for the local processing of motor control based on the external and proprioceptive somatosensory inputs, the connectivity of most sensory axons in the nerve cord remains unknown. In addition, little is known about the organization of the sensory pathways from the nerve cord to the brain. To investigate these, we categorized sensory afferents in the nerve cord of the fruit fly based on their sensory modality and axon branch morphology, and identified matching neurons in the Male Adult Nerve Cord (MANC) electron microscopy (EM) connectome. Unambiguous identification of EM neuron data will enable precise analysis of modality-specific sensory signal pathways. To investigate the sensory axons projecting further from the nerve cord to the brain, we used stochastic labeling and obtained >300 segmented light-microscopy neuron images. This enabled detailed analysis of axon terminal structures at single-cell resolution for the known types of sensory neurons that project directly to the brain, including leg gustatory neurons, movement-sensing leg proprioceptors (chordotonal organs), and proprioceptors from the wings and halteres (campaniform sensilla). In addition, we found previously-undescribed afferents directly from the internal organs of the abdomen, which terminate in the brain region where no previous sensory inputs have been reported.