Asymmetric distribution of gap junctions amongst identified photoreceptor axons of Lucilia cuprina (Diptera).

The relative number of gap junctions within the ring of axon terminals of identified adult photoreceptors has been counted, in the lamina cartridges of Lucilia cuprina. Terminals are similar in size, but some pairs of terminals make up to four times as many junctions with each other as do other pairs; the frequency of connection varies markedly along the length of the cartridge. Junctions across the cartridge occasionally occur out of the normal sequence, frequently in the upper cartridge between one pair of axons. The distributions suggest that mechanical factors transmitted down from the upper cartridge are responsible for the non-uniformities. There may be indiscriminate competition between terminals at some stage of development for a fixed quota of junctions, the groupings of which have to reach a minimum size to remain viable. Amongst the neurones of the cartridge, great specificity is shown in which cells form gap junctions: only receptors R1-6 are involved, making junctions only with each other. Within this class R1-6, however, there is no evidence from the distribution of gap junctions for unique cell labelling, that distinguishes one photoreceptor from another. Connections appear to form indiscriminately, on the basis of proximity. This same generalization holds true for a particular class of chemical feedback synapse. The identity of tight-junction-like structures on the photoreceptor axon membrane is also discussed.