Analysis of Nematode Ventral Nerve Cords Suggests Multiple Instances of Evolutionary Changes to Neuron Number

Despite their diversity in habitats, nematodes are often considered to have a highly conserved neuroanatomy. This premise is based on only a subset of the nematode phylogenetic tree within the subclass Chromadoria, which includes the model organism Caenorhabditis elegans, thereby limiting our understanding of macroevolutionary trends in nervous system structure. To approach this problem, we used nuclear morphology to quantify the number of neurons in the nematode ventral nerve cord (VNC) across the phylum and identified evolutionary patterns in neuroanatomical organization. Nuclear staining revealed that Dorylaimia has significantly more VNC neuronal nuclei than other taxa in Enoplia and Chromadoria, with some species having four times the number of neurons as C. elegans. These results suggest at least two independent transitions in VNC neuron number across subclasses. To further examine developmental patterns and potential variation in nervous system architecture of species with substantially more neurons than C. elegans, we established an isogenic culture of Mononchus aquaticus (Dorylaimia). We found that while M. aquaticus contained four times as many VNC neuronal nuclei as C. elegans, the VNC had a similar developmental timeline during post-embryonic stages. However, dye-filling assays also revealed an extensive distribution of neurons along the lateral body wall of M. aquaticus, which have no obvious homologs in C. elegans. We further found that M. aquaticus is capable of sustained movement following bisection and speculate that this ability results from a more decentralized neuronal network. Our results provide a roadmap for understanding phylum-wide nervous system evolution and demonstrate large-scale differences in neuroanatomy across the phylum.