Analysis of Nematode Ventral Nerve Cords Suggests Multiple Instances of Evolutionary Addition and Loss of Neurons

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 focused on the more diverged clades within the class Chromadorea, 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) to identify evolutionary patterns in neuroanatomical organization within the basal clades of Chromadorea and Enoplea. DAPI staining revealed significant VNC neuron count variations among families, with notable differences between the classes Enoplea and Chromadorea and among Enoplean species. These results may indicate a degree of evolutionary morphological stasis in later diverging Chromadorean clades. To further examine developmental patterns and potential variation in Enoplean nervous system architecture, we established an isogenic culture of the nematode Mononchus aquaticus. 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 revealed an extensive distribution of neurons along the lateral body wall, which have no obvious homolog to C. elegans. We found that M. aquaticus is capable of sustained movement following bisection that may imply a more distributed nervous system network. Our results provide a roadmap for understanding phylum-wide nervous system evolution and demonstrate large-scale differences between Enoplean and Chromodorean nervous systems.