EVIDENCE FOR SITE SELECTION DURING SYNAPTOGENESIS - THE SURFACE DISTRIBUTION OF SYNAPTIC SITES IN PHOTORECEPTOR TERMINALS OF THE FLIES MUSCA AND DROSOPHILA

1. Photoreceptor terminals in the flies Musca domestica and Drosophila melanogaster have been reconstructed in three dimensions from serial EM to reveal the surface distributions of afferent tetrad synapses. 2. The terminals are cylindrical and surround two target cells; they hav e synaptic sites distributed along their length and around their circu mference, except for a strip along the face that lies furthest away fr om the target cells. 3. Over their inner faces, the terminals have pre synaptic sites that are distributed evenly. 4. The distribution of sit es in maps plotted from reconstructed membrane surfaces was examined b y quadrat analyses, The frequency of sites per quadrat division was no t Poissonian, i.e. was non-random. Thus, some form of site selection m ust exist during synaptogenesis. 5. The sites were shown by variance r atio analysis to be regular (evenly dispersed, not clustered). This su ggests that some form of interaction exists, so as to reduce the proba bility that a synapse will form close to an already existing synaptic site. 6. Distances between nearest-neighbour pairs of synapses had a c losest minimum spacing of about 0.8 mu m in Musca that was violated by about 5% of pairs, whereas the corresponding distances were about 0.2 mu m shorter in Drosophila, which had 13% of pairs situated closer to gether than 0.8 mu m. 7. During synaptogenesis, either initially in th e pupa or later in the adult, the probability that a synapse will form is therefore effectively zero within these distances from an existing synaptic site, perhaps through an inhibitory influence exerted by the latter. The nearest-neighbour distances are normally distributed. 8. Unlike the distribution of presynaptic sites, the distribution of post synaptic sites over the surfaces of the dendrites of the target cells is not even. Although not studied in detail, the corresponding nearest -neighbour distances are much smaller, as little as 0.1 mu m. Thus the wider spacing seen between sites over the receptor terminals is a fun ction of the presynaptic cells, and not of their postsynaptic partners , and implies the existence of interactions between synaptic sites.