RUI: Developmental Biology of Turtle Shell Formation

Swarthmore College, Swarthmore PA

Investigators

Abstract

Scott F. Gilbert Proposal #IOS-0748508 RUI: Developmental biology of turtle shell formation The turtle shell is an evolutionary novelty composed of two main parts, the upper carapace and the lower plastron. What "makes a turtle a turtle" is the migration of the ribs into the dermis of the back skin rather than their migrating downward to form a rib cage. This study will use cell transplantation and gene labeling techniques to identify the molecules secreted by the ribs which cause the skin to turn into bone. We expect that the ribs are secreting proteins called Bone Morphogenetic Proteins (BMPs) and that the timing of bone formation is regulated by the interplay between these BMPs and inhibitors of BMPs that exist in the dermis. Cells forming the plastron originate in an embryonic structure called the trunk neural crest. The trunk neural crest has not been seen to produce bones on any other extant species. The cranial neural crest, however, usually makes facial bones. If trunk neural crest cells lose the expression of Hox genes, they simultaneously gain the ability to form skeleton. Hox gene expression appears to inhibit the ability of neural crest cells to form bone. So another major objective of this project is to test the hypothesis that in turtle embryos, a population of neural crest cells is produced late, loses its Hox expression, and migrates to form the plastron bones. If these hypotheses are validated, this research goes far in explaining the "age-old" question of how the turtle got its shell. It would provide a solution to the question of how a novel anatomical structure can evolve so quickly in the fossil record, and it would also provide a readily understood example how evolution works to make new structures. It will be done by undergraduates and should acquaint them with the basic techniques of evolutionary developmental genetics.