REGULATED SPACING OF SYNAPSES AND PRESYNAPTIC ACTIVE ZONES AT LARVAL NEUROMUSCULAR-JUNCTIONS IN DIFFERENT GENOTYPES OF THE FLIES DROSOPHILAAND SARCOPHAGA

Synapses at larval neuromuscular junctions of the flies Drosophila mel anogaster and Sarcophaga bullata are not distributed randomly. They ha ve been studied in serial electron micrographs of two identified axons (axons 1 and 2) that innervate ventral longitudinal muscles 6 and 7 o f the larval body wall. The following fly larvae were examined: axon 1 -wild-type Sarcophaga and Drosophila and Drosophila mutants dunce(m14) and fasII(e76), a hypomorphic allele of the fasciclin II gene; and ax on 2-drosophila wild-type, dunce(m14), and fasII(e76). These lines wer e selected to provide a wide range of nerve terminal phenotypes in whi ch to study the distribution and spacing of synapses. Each terminal va ricosity is applied closely to the underlying subsynaptic reticulum of the muscle fiber and has 15-40 synapses. Each synapse usually bears o ne or more active zones, characterized by dense bodies that are T-shap ed in cross section; they are located at the presumed sites of transmi tter release. The distribution of synapses was characterized from the center-to-center distance of each synapse to its nearest neighbor. The mean spacing between nearest-neighbor pairs ranged from 0.84 mu m to 1.05 mu m for axon 1, showing no significant difference regardless of genotype. The corresponding values for axon 2, 0.58 mu m to 0.75 mu m, were also statistically indistinguishable from one another in termina ls of different genotype but differed significantly from the values fo r axon 1. Thus, the functional class of the axon provides a clear pred iction of the spacing of its synapses, suggesting that spacing may be determined by the functional properties of transmission at the two typ es of terminals. Individual dense bodies were situated mostly at least 0.4 mu m away from one another, suggesting that an interaction betwee n neighboring active zones could prevent their final positions from be ing located more closely. (C) 1998 Wiley-Liss, Inc.