Graph theoretical analyses of nervous systems usually omit the aspect of connection
polarity, due to data insufficiency. The chemical synapse network of Caenorhabditis
elegans is a well-reconstructed directed network, but the signs of its connections
are yet to be elucidated. Here, we present the gene expression-based sign prediction
of the ionotropic chemical synapse connectome of C. elegans (3,638 connections and
20,589 synapses total), incorporating available presynaptic neurotransmitter and postsynaptic
receptor gene expression data for three major neurotransmitter systems. We made predictions
for more than two-thirds of these chemical synapses and observed an excitatory-inhibitory
(E:I) ratio close to 4:1 which was found similar to that observed in many real-world
networks. Our open source tool (http://EleganSign.linkgroup.hu) is simple but efficient
in predicting polarities by integrating neuronal connectome and gene expression data.
