Hyoid Neural Crest Migration and the Evolution of Tetrapod Novelties

What led to the profound evolutionary transformation of an ancestral fish head into a mammalian one? How does a filter-feeding tadpole turn into a frog with a tongue and a new set of sensory, motor and respiratory structures during one lifetime? The underlying mechanisms of these dramatic changes in deep time and during a single ontogeny are unknown despite more than a century of work. Comparative anatomy already recognized how remarkably conserved the osteichthyan skull pattern is, enabling researchers to homologize bones between fish and humans. In recent decades, a deep conservation of underlying developmental signalling pathways and molecular controls have been revealed across craniates. While molecular changes in the strength of signalling systems have been found to explain cranial diversity in smaller taxonomic groupings (such as Darwin finches), large transformations have remained off limits and lineages have not been systematically studied to play a role here. Where cranial lineages were studied carefully (i.e. in chick), again a remarkable conservation of the underlying skeletomuscular patterns mediated by the discrete cell populations was found but not tested systematically across a range of evolutionarily indicative species. Do signalling systems change in a stable lineage population architecture or do lineages move in ontogenetic (and phylogenetic) time and act as mediators of anatomical diversity in space and time? In this thesis I will test this by tracing the developmental and evolutionary history of the hyoid crest and its adjacent mandibular neural crest lineage in zebrafish, mouse, urodele and chick, using novel experimental tools. I reveal a novel time window across fish and tetrapod ontogeny when hyoid NCCs move rostrally and caudally, away from their early hyoid arch location of the so-called phylotypic stage. I will trace spatially discrete streams of these cells, leading to reproducible cryptic boundaries within the snout region, significantly contributing to a host of anatomical structures, muscular and sensory. Further I will see how these cryptic cell population boundaries change during ontogeny, establishing new connections between muscular and skeletal elements with profound consequences for cranial biomechanics. A similar rostral trajectory of the hyoid crest is also found in mice and other tetrapods studied (urodele, chick), giving rise to compound skeletal structures of mandibular and hyoid origin. This sheds new light onto the aetiology of craniofacial defects (such as in Down syndrome) and cleft palates as well as morphometric changes of skulls at the fish-tetrapod transition. Unexpectedly, I also find a significant hyoid contribution to the hitherto unmappable tongue in the mouse and observe how the hyoid NCCs partition the mandibular tongue anlagen. During these transformations, the underlying patterns of skeletomuscular connectivity (i.e. the sameness of attachment region and attached muscle connective tissue) is maintained, but cell populations migrate and carry connectivity codes to new regions. Tracing homologous cell populations across taxa and ontogenies allows me to reveal unexpected homologies across hypobranchial muscles and also shows that the neural crest architecture of the head-shoulder boundary changes at the FTT (from hyoid arch to mandibular arch). The timing of hyoid migration is different between zebrafish and the tetrapods studied, suggesting a remarkable lineage-specific heterochronic event that likely occurred in tetrapod ancestors, an acceleration of rostral hyoid migration and loss of caudal migration, giving rise to a host of anatomical novelties such as choanae, tongue, Jacobson’s organ, gill cover loss and remodelling of the head-shoulder connection. This comparative exploration of the hyoid crest lineage during ontogeny provides a new paradigm to study the developmental origins of cranial anatomical novelty at the molecular level: a remarkable cell population as an unexpected driver of evolutionary change