Subsets of leg proprioceptors influence leg kinematics but not interleg coordination in Drosophila melanogaster walking

Legged locomotion in terrestrial animals is often essential for mating and survival, and locomotor behavior must be robust and adaptable to be successful. This adaptability is largely provided by proprioceptors monitoring positions and movements of body parts and providing feedback to other components of locomotor networks. In insects, proprioceptive chordotonal organs span joints and encode parameters of relative movement between segments. Previous studies have used whole-organ ablation, reduced preparations or broad physiological manipulations to impair the function of the femoral chordotonal organ (fCO), which monitors the femur-tibia joint, and have demonstrated its contribution to interleg coordination and walking behavior. The fCO in Drosophila melanogaster comprises groups of neurons that differ in their morphology and encoding properties (club, hook, claw); sub-population-level manipulations of fCO function have not been methodologically accessible. Here, we took advantage of the genetic toolkit available in D. melanogaster to identify sub-populations of fCO neurons and used transient optogenetic inhibition to investigate their roles in locomotor coordination. Our findings demonstrate that optogenetic inhibition of a subset of club and hook neurons replicates the effects of inhibiting the whole fCO; when inhibited alone, however, the individual subset types did not strongly affect spatial aspects of single-leg kinematics. Moreover, fCO subsets seem to play only a minor role in interleg temporal coordination. Thus, the fCO contains functionally distinct subgroups, and this functional classification may differ from those based on anatomy and encoding properties; this should be investigated in future studies of proprioceptors and their involvement in locomotor networks.