Variation in larval locomotion and NMJ among melanogaster sibling species. Emma Yang, Mirela Belu, Claudia Mizutani. Case Western Reserve University, Cleveland, OH.

   Closely related Drosophila species are ideal models for studying evolutionary processes given the genomic resources available and the expected small number of genes responsible for the emergence of new phenotypes. A first step to begin to explore these questions is the identification of phenotypic variation among species. Here we describe clear phenotypic variations in the locomotion patterns and Neuromuscular Junction (NMJ) within the sibling species D. melanogaster, D. simulans and D. sechellia. By using a video-tracking analysis, we extracted parameters of larval forward crawling of speed, rate of contraction and dispersal efficiency. These analyses revealed that D. melanogaster and D. simulans have similar locomotion patterns, while the more recently diverged species D. sechellia has a lower contraction rate, speed and dispersal. The similar locomotion patterns observed in D. melanogaster and D. simulans correlate well their NMJ anatomy, with similar numbers of active synaptic boutons and branching points per muscle area. In contrast, D. sechellia contains a smaller number of active boutons and branchings. In addition, D. sechellia is the only species with octopamine+ type II boutons in muscle fibers 6/7, an inhibitory neurotransmitter. To test the genetic contribution to these phenotypes, we generated and analyzed the locomotor patterns of hybrid larva between these species. These experiments indicate that some parameters of locomotion and the formation of type II boutons of D. sechellia are X-linked. Based on these results, we are using bioinformatics tools to search for fast evolving genes located on the X chromosome that are most conserved between D. melanogaster and D. simulans, but most divergent in D. sechellia. Currently, we are testing five candidate genes obtained through the searches, and exploring their roles in NMJ development.