Getting a head start: Craniofacial heterochrony in marsupials involves dynamic changes to molecular and cellular mechanisms underlying neural crest development

The neural crest is a vertebrate innovation central to craniofacial development and evolution. While the gene regulatory networks guiding neural crest development are well characterized, the mechanisms generating species-specific craniofacial diversity remain poorly understood. Marsupials provide a unique model for studying neural crest plasticity, having evolved accelerated patterns of craniofacial development during embryogenesis. This adaptation arises in response to marsupials being born altricial after a short gestation yet require well-developed mouthparts to attach to a teat and continue development in the pouch. However, how marsupials achieve this heterochronic shift in neural crest development is largely unknown. In this study, we investigate the cellular and molecular mechanisms underlying their distinct heterochrony, revealing that marsupials produce dense pre-migratory aggregates of neural crest cells which undergo collective migration as epithelial-like sheets, potentially facilitating rapid establishment of the facial prominences. These cellular behaviours are unique amongst amniotes but resemble patterns in anamniotes which similarly exhibit accelerated craniofacial development to support early feeding. Marsupials appear to have evolved a similar mechanism of neural crest migration to facilitate their developmental heterochrony. These findings suggest that vertebrate neural crest migration may be shaped by the pace of craniofacial development during embryogenesis rather than phylogeny, providing new perspectives on neural crest plasticity and the developmental mechanisms driving craniofacial diversity across vertebrates.