UBC Theses and Dissertations
How the turtle makes its palate without palatal shelves Leung, Kelvin Jia-Mien
Vertebrate craniofacial development and speciation has been studied in great detail, with major emphasis placed on mammalian species and highly derived archosaurs (birds). However, less is known about reptiles and in particular turtles. Turtles are speculated as to have retained many ancestral features of amniotes. Therefore, studying the Testudine (turtle) order not only helps to better understand amniote head development, but also the derivation of modern form. This thesis will investigate the formation of the hard palate in a representative turtle species, E. subglobosa, not only because of its evolutionary significance but also because this region is frequently affected in orofacial clefting. Origins of the palatine bones were first examined since other amniotes form these bones within outgrowths of the maxillary prominence, or the palatal shelves. Surprisingly no palatal shelves were found at the position or time when they should have been forming. Instead palatine bones condensed directly in the mesenchyme beneath the nasal cavity Furthermore there was no evidence from cell proliferation or apoptosis analysis of the maxillary prominences that vestigial shelves were ever present. The hypothesis following was that gene expression in the maxillary prominences might be different in turtles compared to the chicken or mouse in which shelves do form. I found no major differences but interestingly several of the genes I studied were also markers of the primitive stomodeum. Results show the turtle retains gene expression patterns of the chicken stomodeum, the primitive oral roof before palatal shelf formation, suggesting the turtle oral roof is still primitive in nature rather than advanced in other amniotes. This unfamiliar mechanism of hard palate development with no vestigial traits of palatal shelf formation supports arguments for a more basal placement of the turtle in the phylogenetic tree. Contrary to these findings, the similarity in gene expression and sequence to the chicken argues for a more derived placement closer to the archosaurs. While these present results do not allow for confident placement of the turtle as more basal or derived in the amniote tree, the data collected shows that ontological studies can help shed light on evolutionary debates.
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