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Title: Evolution of the amphibian head and neck: fate and patterning of cranial mesoderm in the axolotl (Ambystoma mexicanum)
Abstract: The vertebrate head is a complex structure derived from all three embryonic germ layers. Cranial mesoderm forms most of the neurocranium, cardiovascular tissues and voluntary muscles required for intake of food and oxygenated fluid. Despite its essential role in shaping cranial and neck anatomy, long-term fate maps of cranial mesoderm are known only from the mouse and chicken, as effective labeling techniques for use in other species have been developed only recently. Data from additional species are needed to determine the embryonic origin of features absent in amniotes but present in other vertebrates and to evaluate the extent of conservation in the development of homologous structures. This dissertation examines the role of cranial mesoderm as well as its interactions with neural crest in shaping the tetrapod craniofacial and neck region, focusing on the skull and head muscles in the axolotl, Ambystoma mexicanum. I demonstrate a dual embryonic origin of the pharyngeal skeleton, including derivation of basibranchial 2 from mesoderm closely associated with the second heart field. Additionally, heterotopic transplantation experiments reveal lineage restriction of mesodermal cells that contribute to pharyngeal cartilage. The entire parietal bone is derived from mesoderm. Several structures arise from both mesoderm and cranial neural crest, including the squamosal, parasphenoid and stapes. The mesodermal contribution to the dorsal portion of the squamosal bone supports the homology of the corresponding dorsal ossification center, which fuses to the ventral center early in development, to the supratemporal, a bone lost repeatedly in tetrapods. I locate the posterior limit of myogenic cranial mesoderm, extending the head-trunk boundary to the axial level of the third somite. Using fate mapping, gene expression and comparative anatomy, I provide evidence that the cucullaris muscle, a homologue of the mammalian trapezius, is a cranial muscle allied with the gill levators of anamniotes. Finally, I generate two novel transgenic lines of Xenopus tropicalis that will be used to fate map neural crest and mesoderm. Taken together, these results add to our understanding of cranial homologies and point to a larger role for cranial mesoderm in the evolution of a mobile neck.