Reconstruction of CSDn partners in 9 glomeruli

We chose 9 glomeruli to reconstruct all CSDn synaptic partners in the contralateral AL based on the lifetime sparseness of the glomerulus as well as the number of presynaptic and postsynaptic sites of the CSDn (see Fig. 2A). To reconstruct synaptic partners in specific glomeruli, the completed reconstruction of the CSDn was filtered in CATMAID so that only CSDn branches and synapses within a specific glomerulus were visible, using glomerulus volume meshes generated by Bates et al. (2020b). All neurons synapsing on the CSDn or receiving synaptic input from the CSDn were then reconstructed in a given glomerulus toward the neuron's primary neurite (i.e., backbone) and ceased when the reconstruction was sufficient to identify the neuron as a PN, LN, ORN, or other neuron type based on its soma location and/or projection pattern. Short neuron fragments that could not be reconstructed to backbone because of ambiguity of projections were deemed “orphans” and were excluded from analyses. Reconstructions were reviewed from starting synapse to the backbone (primary neurite) by a second expert tracer and then queried using NBLAST as needed.

Glomerulus-specific innervation by the CSDn. A, Glomeruli rank-ordered by the total number of CSDn synaptic sites. The number of CSDn presynaptic sites (red bars) and postsynaptic sites (blue bars) are correlated to total CSDn cable length in each glomerulus (black diamonds) (R² = 0.8231). B, Input/output index of the CSDn in each glomerulus is fairly consistent across glomeruli (mean = 0.4359, SEM = 0.01, coefficient of variation = 22.96%). CSDn branch length in glomeruli is highly correlated to (C) CSDn presynaptic sites (R² = 0.924, p < 0.0001) and (D) number of CSDn postsynaptic sites (R² = 0.861, p < 0.0001). E, CSDn presynaptic density (red) (# sites/nm branch length) and postsynaptic density (blue) are fairly consistent across glomeruli (coefficient of variation = 26.26% and 30.85%, respectively), although postsynaptic is more distributed (p < 0.005, Levene's test for homogeneity of variance). F, CSDn branch length per glomerulus weakly correlates to glomerular volume (R² = 0.201, p = 0.001).