Abstract
During the development of peripheral nerves, pioneer axons often navigate over mesodermal tissues. In this paper, we examine the role of the mesodermal cell determination gene tinman on cells that provide pathfinding cues in Drosophila. We focus on a subset of peripheral nerves, the transverse nerves, that innervate abdominal segments. During wildtype embryonic development, the transverse nerve efferents associate with glial cells located on the dorsal aspect of the CNS midline (transverse nerve exit glia). These glial cells have cytoplasmic extensions that prefigure the transverse nerve pathway from the CNS to the body wall musculature prior to transverse nerve formation. Transverse nerve efferents extend along this scaffold to the periphery, where they fasciculate with projections from a peripheral neuron--the LBD. In tinman mutants, the transverse nerve exit glia appear to be missing, and efferent fibers remain stalled at the CNS midline, without forming transverse nerves. In addition, fibers of the LBD neurons are often truncated. These results suggest that the lack of exit glia prevents normal transverse nerve pathfinding. Another prominent defect in tinman is the loss of all dorsal neurohemal organs, FMRFamide-expressing thoracic structures which likely contain the homologs of the transverse nerve exit glia in the thoracic segments. Our results support the hypothesis that the exit glia have a mesodermal origin and that glia play an essential role in determining transverse nerve axon pathways.