Synaptogenesis in the Giant-Fibre System of Drosophila: Interaction of theGiant Fibre and its major motorneuronal target

The tergotrochanteral (jump) motorneuron is a major synaptic target of the Giant Fibre in Drosophila, These two neurons are major components of the fl y's Giant-Fibre escape system, Our previous work has described the developm ent of the Giant Fibre in early metamorphosis and the involvement of the sh aking-B locus in the formation of its electrical synapses. In the present s tudy, we have investigated the development of the tergotrochanteral motorne uron and its electrical synapses by transforming Drosophila with a Gal4 fus ion construct containing sequences largely upstream of, but including, the shaking-B(lethal) promoter. This construct drives reporter gene expression in the tergotrochanteral motorneuron and some other neurons. Expression of green fluorescent protein in the motorneuron allows visualization of its ce ll body and its subsequent intracellular staining with Lucifer Yellow. Thes e preparations provide high-resolution data on motorneuron morphogenesis du ring the first half of pupal development. Dye-coupling reveals onset of gap -junction formation between the tergotrochanteral motorneuron and other neu rons of the Giant-Fibre System. The medial dendrite of the tergotrochantera l motorneuron becomes dye-coupled to the peripheral synapsing interneurons between 28 and 32 hours after puparium formation. Dye-coupling between terg otrochanteral motorneuron and Giant Fibre is first seen at 42 hours after p uparium formation, All dye coupling is abolished in a shaking-B(neural) mut ant, To investigate any interactions between the Giant Fibre and the tergot roachanteral motorneuron, we arrested the growth of the motorneuron's media l neurite by targeted expression of a constitutively active form of Dcdc42. This results in the Giant Fibre remaining stranded at the midline, unable to make its characteristic bend. We conclude that Giant Fibre morphogenesis normally relies on fasciculation with its major motorneuronal target.