Topographical Projections of Limb-Innervating Motor Neurons in Drosophila melanogaster Specified by Morphological Transcription Factors and Downstream Cell Surface Proteins

The nervous system integrates multiple sources of sensory information that ultimately controls motor neurons to generate complex movements. Motor neurons form topographically organised ‘myotopic maps’ between the nerve cord and muscles in the periphery to ensure that correct pre-motor inputs into motor dendrites are relayed through corresponding axons to the appropriate muscle groups. Therefore understanding the development and assembly of motor neuronss is crucial for understanding how animals execute various motor outputs. In adult Drosophila, ~50 motor neurons are topographically organized between each leg and the nerve cord in a highly stereotyped manner (Baek and Mann, 2009). In this thesis, I describe a novel group of transcription factors that act in a combinatorial manner to specify the projections of distinct Drosophila leg motor neurons. Our studies suggest that morphological transcription factors regulate various downstream cell-surface genes that are involved in the assembly of motor circuitry. Using in vivo live imaging I describe the developmental steps involved in Drosophila leg motor neuron axon targeting during metamorphosis and the spatial expression patterns of a novel hetero-binding Ig domain transmembrane protein family – the DIPs and Dprs (Ozkan et al., 2013) in leg neuro-musculature. I further describe a function between interacting partners DIP-alpha and Dpr10, expressed in subsets of leg motor neurons and muscles respectively, in establishing the final stereotyped terminal axon branching of corresponding motor neurons. The combinations of such interactions throughout development between leg motor neurons, not only with muscles in the periphery, but also among themselves, with leg sensory neurons and other components in the central nervous system may ultimately lead to synaptic specificity and stereotyped morphologies of Drosophila leg motor neurons.