The rewiring of a terminal selector regulatory cascade generates convergent neuronal laterality

Neuronal identity is established and maintained by "terminal-selector" transcription factors, yet how these networks evolve remains unclear. We examined the specification of the chemosensory ASE and thermosensory AFD neurons in the nematode Pristionchus pacificus, a species that expresses the terminal-selector, Ppa-CHE-1, in both sensory neurons. To determine if the ASE neurons exhibit left-right laterality, we used HCR-FISH and transgenic reporters to discover 8 ASE left-right-specific and 3 AFD-specific receptor-type guanylyl-cyclases. Late embryos exhibit a multipotential state in which AFD precursors transiently co-express all three types of ASEL, ASER and AFD markers. A forward genetic screen for defects in ASER asymmetry identified a Ppa-DIE-1 homolog, whereas targeted mutations revealed the maintenance of AFD neuronal identity requires another terminal-selector, Ppa-TTX-1, and CNG channels, Ppa-TAX-2/TAX-4. Mutations in the microRNA miR-8345 and pash-1 responsible for miRNA-processing convert ASEL to ASER fate while changes to other conserved regions in the 3' UTR of the cog-1 homolog reveal multiple sites that act as a toggle between left/right ASE versus AFD identities. Together, these results demonstrate that P. pacificus deploys a miRNA-mediated regulatory repertoire to generate three distinct neuronal fates through the Ppa-cog-1 3' UTR as a key regulatory nexus.