MOUSTAKAS, J.E.; University of California, Berkeley: Developmental genetics of dermal ossifications in turtles and alligators

Developmental genetics has provided us new ways to investigate homology. We can more fully understand the level of homology being addressed by using other lines of evidence to test our hypotheses. The example I have chosen to investigate is the homology of dermal ossifications in reptilian lineages. Testudinians and crocodilians have dermal bones in their trunk regions that ossify directly from mesenchymal cells rather than through cartilaginous precursors. These bones have been proposed to originate from an embryonic population of neural crest cells, and to be homologous to dermal ossifications in the trunk regions of basal vertebrates. The neural crest gives rise to the dermal bones of the skull, and contributes tissues to several other systems of the vertebrate body. In normal development, these cells have never been shown to give rise to postcranial bone tissue in vertebrates thus far investigated. Because the embryonic origin of these tissues in reptiles is difficult to determine, I have begun to investigate the homology of these tissues by looking at several regulatory genes involved in the development of skeletal tissues. Given that crocodilian scutes and turtle shells are both ossifications formed in the dermis, like the dermal bones of the cranium, do they also originate from neural crest cells? If so, this would imply a completely new role and distribution of neural crest cells. And if not, what governs the development of this tissues? What would a different embryonic origin, yet similar regulatory gene expression, imply for the homology of dermal bones?