Evidence of wiring development processes from the connectome of adult Drosophila

How is the brain wired during development? Here we look at some of the characteristics of the adult Drosophila brain, a result of this development process. From this we can speculate on at least some aspects of how the wiring was done. A common hypothesis is that surface proteins direct synapse formation among touching neurons. Assuming that surface proteins are specific to cell types, we find support for this hypothesis. The brain in general supports a wide variety of connections of different degrees of reciprocity, with ratios up to a thousand to one in the two directions of synapses between types. However, contacts between cells of the same type are always bi-directional and nearly equal strength. Furthermore, among similar cell types, at least in the mushroom body, the closer the cell type by morphology, the closer the inter-type contacts are to symmetrical. These findings are all consistent with the hypothesis that surface proteins specific to cell types determine the directivity of connections. Next we look at synapses per area, and find this varies widely and is roughly log-normally distributed. In most cases, the number of synapses saturates at higher areas, though other forms are seen - linear, flat, or decreasing with increasing area. Evidence suggests that at least some of this distinction is post-synaptic.