NEURAL BASIS OF LOCOMOTION IN DROSOPHILA MELANOGASTER LARVAE
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Drosophila larval crawling is an attractive system to study patterned motor output at the level of animal behavior. Larval crawling consists of waves of muscle contractions generating forward or reverse locomotion. In addition, larvae undergo additional behaviors including head casts, turning, and feeding. It is likely that some neurons are used in all these behaviors (e.g. motor neurons), but the identity (or even existence) of neurons dedicated to specific aspects of behavior is unclear. To identify neurons that regulate specific aspects of larval locomotion, we performed a genetic screen to identify neurons that, when activated, could elicit distinct motor programs. We defined 10 phenotypic categories that could uniquely be evoked upon stimulation, and provide further in depth analysis of two of these categories to understand the origins of the evoked behaviors. We first identified the evolutionarily conserved Even-skipped+ interneuron phenotype (Eve/Evx). Activation or ablation of Eve+ interneurons disrupted bilaterally symmetric muscle contraction amplitude, without affecting left-right synchronous timing. TEM reconstruction places the Eve+ interneurons at the heart of a sensorimotor circuit capable of detecting and modifying body wall muscle contraction We then went on to identify a unique pair of descending neurons dubbed the ‘Mooncrawler’ descending neurons (McDNs) to be sufficient to generate reverse locomotion. We show that the McDNs are present at larval hatching, function during larval life, and are remodeled during metamorphosis while maintaining basic morphological features and neural functions necessary to generate backwards locomotion. Finally, using serial section Transmission Electron Microscopy (ssTEM) to map neural connections to upstream and downstream elements provides a mechanistic view of how sensory information is received by the McDNs and transmitted to the VNC motor system to perform backwards locomotion. Finally, we show that these McDNs are the same as those identified in recent work in Drosophila adults (Bidaye et al. 2014) to be sufficient to generate reverse locomotion. This dissertation includes previously published, co-authored material.