The turtle shell is an evolutionary novelty representing a unique departure from the usual way that vertebrate skeletons develop. The turtle shell is a casing made almost exclusively of bone, and it is composed of two main parts, the dorsal carapace and the ventral plastron. Between them, on the lateral sides, is a bridge. The carapace contains fifty bones and is supported by the ribs, which grow dorsolaterally rather than ventrally. The critical event in turtle development and evolution is thought to be the "ensnarement" of the ribs by the carapacial ridge, a bulge of ectoderm and mesoderm that resembles the limb bud. The bones of the shell are thought to be formed from the dermis by intramembranous ossification, with contributions from these ribs. Dr. Gilbert wish to test three hypotheses for the turtle shell development: (1) that the carapace ridge forms by the co-option of the developmental program that forms the limb buds; (2) that the ribs induce intramembranous ossification in the dermis; and (3) that the dorsal growth of the rib cartilage precursors depends on their encountering the carpacial ridge. To test the first hypothesis, He will examine in the carapacial ridge the spatiotemporal expression profile of genes (especially those encoding fibroblast growth factors) known to initiate limb morphogenesis. To test whether the ribs induce intramembranous ossification, He will look at the expression, in the ribs and in other regions, of those genes encoding paracrine factors (especially the bone morphogenetic proteins) that could induce the ossification of the dermis. He will also insert beads containing inhibitors of known bone-inducing paracrine factors to see if the inhibition of these proteins inhibits regional bone development. The behavior of turtle ribs from turtle somites transplanted into chick embryos will tell us if the turtle ribs need the carapacial ridge to grow dorsolaterally or if this is an intrinsic property of the rib cartilage precursors. The results of this research should give insights into the mechanisms by which the co-option of developmental instructions can be used to create evolutionarily novel structures. It should also aid in the reconstruction of the phylogenetic path leading to the "sudden" emergence of turtles in the Triassic.
