The
fax-1 nuclear hormone receptor and
unc-42 homeobox gene control interneuron identities in C. elegans. Both transcription factors function in specifying the identities of an overlapping subset of nematode interneurons including AVA, AVE, and AVK. Both
fax-1 and
unc-42 mutations cause a novel peri-hatching arrest in combination with
daf-2 insulin receptor mutations. Mutations in both
fax-1 and
unc-42 also enhance other aspects of insulin pathway function, including dauer formation and adult arrest, but not longevity, suggesting that disrupting the differentiation of specific interneurons leads to a reduction in insulin signaling. Peri-hatching arrest can be reversed by a mutation in the
daf-16 forkhead transcription factor, which functions downstream of
daf-2, but not by mutations in the parallel TGF? pathway. Arrest can also be suppressed by mutations in the
daf-12 nuclear receptor, indicating that the canonical insulin and steroid pathways that regulate dauer formation also promote arousal and developmental progression at hatching. Arrested
fax-1;
daf-2 and
unc-42;
daf-2 embryos typically display normal L1 morphology, but often remain coiled in a broken eggshell in a state of quiescence with weak or absent pharyngeal pumping. Arrested embryos can be prompted to vigorous movement by stimulation with blue light. Temperature-shift experiments suggest that embryogenesis, not L1 feeding after hatching, is the critical stage for quiescence and arrest. This phenotype is similar to that observed when embryos are subjected to osmotic stress. The
ssu-1 sulfotransferase gene is expressed in the ASJ sensory neurons and is required for peri-hatching arrest in response to NaCl, which is also dependent on insulin signaling (Burton et al., 2018). The peri-hatching arrest of both
fax-1;
daf-2 and
unc-42;
daf-2 double mutants is suppressed by a mutation in
ssu-1. This result places the interneuron peri-hatching arousal function downstream of the osmotic stress signal and a putative steroid signal produced by the ASJ interneurons. Several models are consistent with the combined action of a steroid signaling pathway and neuroendocrine functions of insulin signaling in promoting arousal and developmental progression at hatching.