Embryonic regulation and post-embryonic function of the single-minded gene in the Drosophila central nervous system

The single-minded (sim) gene in Drosophila melanogaster has long been known to play important roles in specifying the mesectodermal cell fate in the embryonic central nervous system (CNS). Mesectoderm cells differentiate into CNS midline cells by mid-embryogenesis. CNS midline cells contribute both neurons and glia to the developing nervous system, and are a source of both attractive and repulsive axonal guidance cues that combinatorially pattern the bilateral CNS. Removal of sim function leads to the failure of midline cell formation, and concomitantly a lack of instructive signal presentation to pathfinding axons from the lateral CNS. As a result, commissural axon tracts that cross the midline are largely absent and parallel longitudinal axon tracts that flank the midline appear fused as a single connective at the embryonic mid-plane. Due to this patterning defect, sim mutants are late embryonic lethal. In addition to the CNS midline, Sim can also be found in the developing foregut, posterior terminal structures, and a subset of myoblasts, although its roles in these compartments are less well understood. In collaboration with others, we demonstrated functions for sim in developing posterior terminal structures and gonads, patterning the larval cuticle, organizing the adult brain, and in adult behavior and locomotion. Using the MARCM strategy for positively marking sim mutant cell clones, we demonstrated that in contrast to its role in neurogenesis in the CNS mesectoderm, sim functions to pattern axon fascicles in the larval central brain, a region known to be important in the interhemispheric communication and the coordination of leg movement. Using RT-PCR, we showed that the sim locus yields a third, previously unidentified transcript that is the primary isoform used post-embryonically. Genetic dissection of inter- and intragenic regions from the sim locus revealed locations of enhancers that drive expression in the CNS midline, myoblasts, and foregut. Taken together, these results have broadened our understanding of sim, an important regulator of development with complex regulation.