SINGLE NEURON MOSAICS OF THE DROSOPHILA-GIGAS MUTANT PROJECT BEYOND NORMAL TARGETS AND MODIFY BEHAVIOR

gigas is a lethal mutant that differentiates enlarged cells, including the nucleus. This trait manifests only after the completion of the mi totic program. We have taken advantage of this phenotype to test in vi vo the capacity of normal target cells to arrest the growth of mutant sensory axons. Single neuron connectivity changes have been analyzed i n mosaics after horseradish per oxidase retrograde tracings. A mutant mechanoreceptor neuron, growing over a genetically normal substrate, c ontacts its normal target, and in addition projects to novel areas of the CNS. The mutant axon does terminate its growth eventually, and the new additional targets that are reached correspond to mechanoreceptor domains in other ganglia, indicating that this territorial constraint is operational in the mutant. gigas neurons maintain their stereotype d profile and represent an expanded version of the normal branching pa ttern. The ultrastructure of the invading projections does not reveal gliotic or necrotic reactions from the new cell contacts. The function al consequences of the connectivity changes produced by the mutant mec hanoreceptors have been studied in grooming behavior. Mosaic flies car rying a single gigas mechanoreceptor show modified, albeit context-coh erent, grooming responses after stimulation of the mutant bristle, whe reas the response from neighboring normal sensory neurons remains unch anged. All of these experiments indicate that target recognition and g rowth arrest are two dissectible processes of neural development, and they highlight the autonomous features of the growth cone during pathf inding.