Cooption and Specialization of Endothelin Signaling Pathways Drove Elaboration of the Neural Crest in Early Vertebrates

Abstract

The neural crest (NC) is a vertebrate-specific embryonic tissue that forms an array of clade-defining adult features. A key step in the formation of these diverse derivatives is the partitioning of NC cells into subpopulations with distinct migration routes and potencies1. The evolution of these developmental modules is poorly understood. Endothelin (Edn) signaling is unique to vertebrates, and performs various functions in different NC subpopulations2-5. To better understand the evolution of NC patterning, we used CRISPR/Cas9-driven mutagenesis to disrupt Edn receptors, ligands, and Dlx transcription factors in the sea lamprey, Petromyzon marinus. Lampreys and modern gnathostomes last shared a common ancestor 500 million years ago6. Thus, comparisons between the two groups can identify deeply conserved and divergent features of vertebrate development. Using Xenopus laevis to facilitate side-by-side analyses, we show here that lamprey and gnathostomes display fundamental differences in Edn signaling function. Unlike gnathostomes, both lamprey Ednrs cooperate during oropharyngeal skeleton development. Furthermore, neither paralog regulates hand transcription factors, which are required for mandible development in gnathostomes. We also identify conserved roles for Edn signaling in dlx gene regulation, pigment cell, and heart development. Together our results illustrate the stepwise neofunctionalization and specialization of this vertebrate-specific signaling pathway, and suggest key intermediate stages in the early evolution of the NC.