Membrane contact explains the higher synapse counts of two-way connections in 7 C. elegans nerve rings; in 2 fly connectomes a 1.2 to 1.3 fold excess stays after coarse controls

Question: are two-way (reciprocal) connections stronger than one-way connections only because large neurons and neurons that touch more make more synapses in both directions? Approach: we fit log synapse count on a two-way indicator with presynaptic and postsynaptic neuron fixed effects and dyadic-robust errors. We added membrane contact area (7 C. elegans nerve rings from Witvliet et al. 2021), cell-type pairs, dominant neuropil and a neuropil-overlap proxy (FlyWire v783 and hemibrain v1.2). Results: in the worm, two-way connections had 1.22 times more synapses with neuron effects only (pooled 95% CI 1.17 to 1.27). Two-way pairs had 2.08 times more contact. With contact as a control, the ratio was 0.95 (0.91 to 0.99), and all 7 animals from birth to adult agreed. After contact, forward and reverse residual counts were not correlated (0.04, CI -0.03 to 0.10; 0.15 before contact control), and two-way pairs occurred 941 times against 922 expected. In the fly, at 5 or more synapses, two-way connections stayed 1.23 (FlyWire) and 1.33 (hemibrain) times stronger with neuron and neuropil effects, 1.22 to 1.30 within cell-type pairs, and 1.24 and 1.32 with the neuropil-overlap proxy. Both hemispheres agreed. Fly forward and reverse residuals correlated at 0.13 to 0.26, similar to the worm before contact control. Cholinergic pairs had the smallest fly excess (1.09). A mouse MICrONS sample had only 7 two-way pairs and gave no conclusion. Limitation: the fly tables have no contact areas, so fine-scale geometry can explain the remaining fly excess. Thus a raw two-way strength excess is not sufficient evidence for activity-dependent strengthening.

connectome · reciprocal connections · synapse count · membrane contact · C. elegans · Drosophila · FlyWire · hemibrain · MICrONS · fixed effects · dyadic-robust errors · structure-function