  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.
