HANKEN, J.*; GROSS, J.B.; Harvard University; Harvard University: Fate maps evolve, too: neural crest derivation of the bony skull in frogs, revealed by transgenic labeling

We utilize a novel transgenic labeling system to assess the contribution of cranial neural crest to the bony adult skull in the clawed frog, Xenopus laevis. This system is suitable for long-term labeling of embryonic cells and for tracing their contribution to adult-specific structures, including those that in metamorphosing amphibians form after a prolonged larval period. Results constitute the first direct evidence of neural crest derivation of most cranial bones in amphibians and represent only the second comprehensive fate map for the vertebrate osteocranium. Neural crest derivation of the bony skull in anurans is more extensive than that reported for any other vertebrate. Crest-derived territory extends caudally to include the entire length of the frontoparietal bone in the skull roof (cranial vault) and the rostral portion of the paired exoccipital bones, which enclose the foramen magnum at the rear of the skull. Additional features that differ from those seen in other vertebrates include a significant contribution from the hyoid crest migratory stream to rostral bones, which generally are regarded as derived from the mandibular crest stream. Apparent variation among species in the embryonic derivation of the bony skull may mean that the neural crest-mesoderm boundary has shifted within the skull during vertebrate evolution, and that a given bone in one species may form from a different precursor cell population than the homologous bone in another species. This would offer an example of evolutionary lability of neural crest biology, which in most other respects is highly conserved among species. Alternatively, these data may indicate errors in the traditional assessment of specific cranial bone homologies among vertebrate classes. Supported by NSF EF-0334846 (AmphibiaTree).