CSDn reconstruction

The left-hand CSD neuron (i.e., soma in the fly's left hemisphere) was originally identified based on the unique ω-shaped projection pattern of its primary process that spans the protocerebrum. The CSDn's identity was later confirmed using NBLAST (Costa et al., 2016) (RRID:SCR_015884) to query the reconstructed skeleton against a dataset of light microscopy images of single neurons (Chiang et al., 2011; Milyaev et al., 2012). The primary process of the CSDn was reconstructed from soma to ipsilateral protocerebrum to contralateral protocerebrum, to contralateral AL. Arborizations from the primary process into the LH and ALs were also reconstructed “to completion” using methods described previously (Zheng et al., 2018). For all CSDn branches traced, all presynaptic and postsynaptic sites, as well as presynaptic and postsynaptic partners were marked. In total, 23,328,903 nm of CSDn cable was reconstructed with 2885 presynaptic sites and 4141 postsynaptic sites marked. It should be noted that an incomplete reconstruction of the CSDn was previously published (X. Zhang et al., 2019). It should also be noted that we identified and partially reconstructed the right-hand CSDn (soma in the fly's right hemisphere; data not shown) and observed two synapses between the two CSDns.