Distinct transcriptional programs define cranial motor neuron subtypes during vertebrate development

Closely related neurons that share core differentiation programs can be difficult to distinguish by gene expression, even when they differ in connectivity and function. Cranial motor neurons ( cMNs) exemplify this challenge, forming discrete nuclei that control behaviors as diverse as eye movement, feeding, facial expression, and regulation of visceral organ function. However, the developmental programs that drive cMN target choices and functional specialization have not been comprehensively studied in any vertebrate. Here, we present an integrated single-cell RNA-sequencing atlas of zebrafish cMN development from 18 to 144 hours post-fertilization, combining FACS-purified cMNs with cMNs computationally extracted from published whole-embryo datasets and perform extensive validation by HCR in situ hybridization. We find that each cranial motor nucleus expresses a distinct transcriptional signature, and in many cases, we identify transcriptional correlates to functional subtypes within individual nuclei as determined by retrograde labeling. We find that identity often precedes axon targeting, indicating that cranial motor neuron fate is genetically specified early in development. These distinct identities are shaped by the intersection of shared function, rhombomere origin, and developmental time. A cross-species comparison reveals a largely one-to-one correspondence between zebrafish and mouse cMN nuclei, indicating that these genetic programs are conserved. Together, this atlas provides a nucleus-resolved molecular framework for understanding cranial motor neuron diversification, and for interpreting human cranial dysinnervation disorders.