A neuron that regulates locomotion makes a potential sensory cilium lying over the C. elegans egg-laying apparatus

The neural circuit for C. elegans egg laying has been studied intensively for decades, yet it is not clear that its known components can account for how egg-laying and locomotion behaviors are coordinated. We found that the two PVP neurons, which release neuropeptides that promote roaming locomotion, make previously-undescribed branches that terminate in large wing-shaped endings directly over the egg-laying apparatus. The PVP branches occur in hermaphrodites but not males and develop during the L4 larval stage when the egg-laying system also develops. The PVP wing is located at the junction between the uterus and the vulva, adjacent to neurons that control egg laying, and surrounded by cells that we found label with a glial marker. The morphology of the PVP wing and its envelopment within possible glial cells are consistent with the hypothesis that the PVP wing is a sensory cilium. Although PVP is reported to express sensory receptor homologs, we have been unable to detect PVP expression of more specific markers of neural cilia, and we have also not detected strong PVP defects in the daf-19 mutant, which does show defects in known neural cilia. The PVPs are extraordinarily sensitive to expression of transgenes, which cause developmental and possibly functional defects in these neurons. This has prevented us from recording or manipulating PVP activity to determine its functional roles. Thus, the intriguing hypothesis that PVP is a sensory neuron that might coordinate egg laying and locomotion will remain speculative until better methods to manipulate PVP can be developed.