Turtle-chicken chimera: an experimental approach to understanding evolutionary innovation in the turtle.

Turtles have a body plan unique among vertebrates in that their ribs have shifted topographically to a superficial layer of the body and the trunk muscles are greatly reduced. Identifying the developmental factors that cause this pattern would further our understanding of the evolutionary origin of the turtles. As the first step in addressing this question, we replaced newly developed epithelial somites of the chicken at the thoracic level with those of the Chinese soft-shelled turtle Pelodiscus sinensis (P. sinensis somites into a chicken host) and observed the developmental patterning of the grafted somites in the chimera. The P. sinensis somites differentiated normally in the chicken embryonic environment into sclerotomes and dermomyotomes, and the myotomes differentiated further into the epaxial and hypaxial muscles with histological morphology similar to that of normal P. sinensis embryos and not to that of the chicken. Epaxial dermis also arose from the graft. Skeletal components, however, did not differentiate from the P. sinensis sclerotome, except for small fragments of cartilage associated with the host centrum and neural arches. We conclude that chicken and P. sinensis share the developmental programs necessary for the early differentiation of somites and that turtle-specific traits in muscle patterning arise mainly through a cell-autonomous developmental process in the somites per se. However, the mechanism for turtle-specific cartilage patterning, including that of the ribs, is not supported by the chicken embryonic environment.