69.4 Sunday, Jan. 6 Understanding of the turtle body plan from developmental perspective NAGASHIMA, H*; KURATANI, S; Center for Developmental Biology, RIKEN, Kobe, Japan; Center for Developmental Biology, RIKEN, Kobe, Japan email@example.com
The skeletal morphology of turtles has provided a possible example of evolutionary novelty, in which topological relationship between the rib cage and scapula is altered. Turtle ribs do not grow ventrally as seen in other amniotes but laterally and form a carapace, the dorsal shell. Thus, the turtle scapula is found ventral to the rib cage, contrary to the situation in other amniotes. Anatomically, the vertebrate trunk consists of the dorso-medially located ‘axial’ domain and the ventro-lateral ‘body wall’. Nowicki et al. proposed another classification, ‘primaxial’ and ‘abaxial’ based on the cell-autonomy of somitic cells (2003). Our developmental and comparative anatomical analyses showed that although the ribs arise as primaxial structures both in chicken and turtle embryos, the turtle ribs are confined within the axial region for the entire developmental process. In both species, ribs form a single layer in the body with muscle plate, primordium of intercostal muscles, and the pectoral girdle is situated outside this layer even after the completion of turtle development. In turtles, however, the layer is secondarily folded in the most ventro-lateral axial domain to cover the girdle dorsally. This implies that turtles satisfy the principle of connection by Geoffroy, but they have experienced different ways of folding that resulted in the altered position of scapula. Observation of the muscles innervated by suprascapularis and long thoracic nerves also supported this interpretation. Thus the turtles provide a new type of novelty involving different ways of emrbyonic folding.