Genetic Studies of Neuronal Development in Drosophila melanogaster

Citation

Green, Steven Haym (1982) Genetic Studies of Neuronal Development in Drosophila melanogaster. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3h8z-9176. https://resolver.caltech.edu/CaltechTHESIS:05162018-092739245

Abstract

The projections into the central nervous system (CNS) of several wild-type and genetically ectopic sensory structures were studied by cobalt filling or silver staining and compared for the purpose of determining what factors guide the growth of the sensory axons. The head bristles all arborize in a similar fashion in the subesophageal ganglion although they reach the target by three different routes depending on their position on the head. This arborization is L-shaped with a longitudinal branch and a medially directed branch that crosses the midline. The antennal projection consists of an olfactory lobe component, organized into glomeruli, and an antennal mechanosensory component which can be further subdivided into three branches, the anteriormost of which is identical to the head bristle projection. The tarsi all have similar U-shaped projections into their segment's neuromere with no ascending, descending or contralateral branches.

Axons of ectopic thoracic bristles on the head may enter the brain or the optic lobes. The routes into the brain taken by the ectopic bristles were initially like those of the normal head bristles but were followed for greater or lesser distances and the region of the subesophageal ganglion that is the target of the head bristles was seldom reached. The terminal arborizations of the ectopic bristle axons were generally irregular regardless of where they were: in the subesophageal ganglion, brain or optic lobes. They resembled neither their normal arborizations in the ventral ganglion nor those of the local head sensilla in the brain.

Axons from antennal legs have a pattern of projection grossly similar to that of wild-type antennae in that the same regions of neuropil were innervated. The non-olfactory lobe components of the antennal leg projection were like those of the antenna. However, the arborization in the olfactory lobe was chaotic and there were adventitious projections from the lobe into adjacent neuropil, particularly the subesophageal ganglion. Some elements of these adventitious projections in the subesophageal ganglion were found consistently in almost every preparation. No element of the projection resembled the leg projection in the ventral ganglion.

The axons of ectopic sensilla can reach a normal target if the distance to it from the new location is sufficiently small: axons from abdominal legs in bxd mutants terminate in normal metathoracic leg sensory neuropil and the axons of antennae misplaced as a result of the mutation ant can enter normal antennal targets.

In summary, axons of ectopic sensilla can't reach their normal targets if they enter the CNS far from those targets which suggests that there are no long range cues for guidance of sensory axons. In the "foreign" part of the CNS the axons of ectopic sensilla do not make projections that resemble their normal ones. They initially take routes characteristic of sensilla in their new location but do not follow them consistently. The exception, antennal leg mechanosensory projections, is likely to be a result of a homology between antennal and leg mechanosensory sensilla. These results suggest the following: insect sensory neurons reach their targets mainly by following local and not long-range cues. The growth of these axons is constrained to specific tracts and it is by these that they are guided over long distances to their targets. Tracts recognized by the axon can be recognized at any point and, as the present study shows, this recognition is required not only at the point of entry but continuously, all along the tract, for guidance of the axon. Guidance by the tract appears to depend on an affinity between the axon and the tract that may also exist between axons and tracts of their segmental or functional homologues. Since axons in foreign neuropil have irregular arborizations characteristic neither of their normal ones nor of those of the local sensilla, the arborization pattern is not a result of an internal branching program alone nor of the axon's milieu directing the branching but must depend on a specific interaction between the axon and its target.

The leg motorneurons were identified and described after HRP backfilling from cut legs. The pattern of their positions differs from segment to segment. The bithorax mutations transform the metathoracic pattern into a mesothoracic pattern, paralleling their effect on the epidermis.

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