Fig. 4.

The modular developmental architecture of the protocerebral bridge. a Schematic frontal section of the brain at a posterior level, visualizing the PB and surrounding neuropil compartments, as well as lineages innervating the PB. PBp1 provides large-field neurons branching throughout the PB but has only very sparse connections outside this compartment (mostly intrinsic neuron; see also Fig. 1a). IbSpsP neurons form a group that most likely belongs to the large lineage DM6 (tract DM6dm). These neurons tile the PB with spatially restricted axonal arbors and have extensive dendritic arbors in the inferior bridge (IB) and superior posterior slope (SPS). Two individual neurons with large-field input to the PB, LPsP and P1-9/OA-AL2i1, belong to as yet not identified lineages with cell bodies in the subesophageal zone (SEZ). b Plots of PB-innervating neurons schematically shown in (a), presented in anterior view. Names of lineages and specific types of PB neurons contained within these lineages are given at the top of each image and the corresponding neuron numbers (for both brain hemispheres) are shown at the bottom. Note that somata are shown only for IbSpsP neurons (arrowhead). For all other neuron types, somata are outside the hemibrain volume. c Heatmap showing neurotransmitter prediction for different neurotransmitters listed along the horizontal axis. d Number of T-bars formed by each neuron in the PB color-coded by lineage and organized by individual neurons. Of note, P6-8P9 and SpsP neurons, in addition to being numerically smaller populations also form fewer T-bars per neuron in the PB compared to their PBp1 sister population Delta7. Dopaminergic LPsP neurons form over three-fold higher number of T-bars in the PB than Delta7. e ECDF of the pairwise connectivity strength in the PB formed by the different neuron types. P1-9/OA-AL2i1 octopaminergic neurons form the fewest and weakest connections. Of the PBp1 population, Delta7 neurons (blue) form the most and strongest connections