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First published online 16 May 2007
doi: 10.1242/dev.002618

Development 134, 2219-2226 (2007)
Published by The Company of Biologists 2007
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On the carapacial ridge in turtle embryos: its developmental origin, function and the chelonian body plan

Hiroshi Nagashima1, Shigehiro Kuraku1, Katsuhisa Uchida2, Yoshie Kawashima Ohya1, Yuichi Narita1 and Shigeru Kuratani1,*

1 Laboratory for Evolutionary Morphology, Center for Developmental Biology (CDB), RIKEN, 2-2-3 Minatojima-minami, Kobe 650-0047, Japan.
2 Sado Marine Biological Station, Faculty of Science, Niigata University, 87 Tassha, Sado, Niigata 952-2135, Japan.

* Author for correspondence (e-mail: saizo{at}cdb.riken.jp)

Accepted 29 March 2007

The chelonian carapace is composed of dorsolaterally expanded ribs; an evolutionary change in the rib-patterning program is assumed to be related to this novelty. Turtle embryos exhibit a longitudinal ridge called the carapacial ridge (CR) on the flank, and its histological resemblance to the apical ectodermal ridge of the limb bud implies its inductive activity in the unique patterning of the ribs. We studied the Chinese soft-shelled turtle, Pelodiscus sinensis, and confirmed by labeling with a lipophilic dye, DiI, that the CR contains the somite-derived dermis and that it is a unique structure among amniotes. Using electroporation of a dominant-negative form of LEF-1, the CR-specific gene, we showed that CR-specific genes function in the growth and maintenance of the CR. Microcauterization or implantation of the CR did not change the dorsoventral pattern of the ribs, and only their fan-shaped pattern was arrested by CR removal. We conclude that the CR is a true embryonic novelty among amniotes and, because of the specific expression of regulatory genes, it functions in the marginal growth of the carapacial primordium, thereby inducing the fan-shaped arrangement of the ribs.

Key words: Turtle, Rib, Somite, Tissue interactions, Signaling molecule, Evolutionary innovation


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© The Company of Biologists Ltd 2007