Patterns of connectivity in a Drosophila nerve.

We investigated the spatial patterns of synaptic profiles in en passant synapses between the premotor axon of a peripherally synapsing interneuron (PAPSI) and a set of individually identifiable motoneuron axons in Drosophila melanogaster. These synaptic profiles are distributed as the axons travel parallel to each other in a bundle; the synapses begin as the axons leave the thoracic ganglion and continue peripherally for 45-65 microm. We found that the number of synaptic profiles per micron length of the motoneuron axons was greatest close to the ganglion; the cumulative distribution of profiles could be fitted to curves of the form f(x) = alpha(1 - e(-beta x)), where x = the distance from the thoracic ganglion, and alpha and beta are constants. The distribution of synaptic profiles was also examined in a mutant strain, Passover (Pas), known to affect connectivity in a pathway that includes the PAPSI. The synaptic profiles between the PAPSI and the motoneuron axons appeared ultrastructurally unremarkable in Pas. Also, the total number of synaptic profiles between the PAPSI and the motoneuron axons did not differ between Pas and wild type flies. However, the distribution of synaptic profiles among the individual motoneuron axons did differ significantly from wild type flies, as did the area of contiguity between the motoneuron axons and the PAPSI, which was much greater in Pas than in wild type flies.