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Abstract

The nervous systems of many nematodes are remarkably similar despite considerable divergence of their genomes. We are examining the evolution of nematode nervous systems by characterizing the nervous system of the nematode Pristionchus pacificus (Ppa) to compare with that of the highly described C. elegans (Cel), which boasts a complete cell lineage, and a complete set of neurons and their connectivities in both sexes (Sulston & Horvitz, 1977; Sulston et al., 1983; White et al., 1986; Cook et al., 2019, Nature 571: 63–71). Within a few years, we and others seek to approach a similar level of characterization for Ppa. For example, the connectivity and likely homologies for head sensory neurons in Ppa have been determined, demonstrating considerable similarities plus a few striking differences, such as the absence of amphid ‘winged’ cilia found in Cel (Hong et al., 2019, eLife 2019;8:e47155). Considerable progress has been made in transgenics, traditional reporters and CRISPR generation of mutants and short knock-ins. To study neuronal specification in Ppa, we have epitope-tagged transcription factor genes to examine their expression patterns, including Ppa orthologs of unc-3, unc-86 and unc-42. We have also found that an antiserum to C. elegans unc-86 works in Ppa, showing a pattern identical to that of the epitope-tagged locus. The pattern of Ppa-unc-86 expression is remarkably similar to that of Cel; one difference is in the absence of clear HSN homologs in hermaphrodites. This is consistent with the absence of HSNs in Ppa seen with anti-serotonin. Other differences in serotonergic neurons suggest possible differences in the role of Ppa-unc-86 in Ppa vs. Cel. In Cel, the ‘terminal selector’ unc-86 plays a role in specifying serotonergic fate (Zhang et al., 2014, Development 141: 422-435). For example, in Cel, the AIM neurons express both unc-86 and serotonin; in Ppa presumptive AIM neurons lack serotonin but still express unc-86. We will present these and other results of our analysis of nervous system similarities and difference in C. elegans and P. pacificus.