Rice Ritva, Riccio Paul, Gilbert Scott F, Cebra-Thomas Judith
Institute of Biotechnology, University of Helsinki, Helsinki, Finland.
J Exp Zool B Mol Dev Evol. 2015 May;324(3):208-20. doi: 10.1002/jez.b.22600. Epub 2015 Feb 11.
Two of the major controversies in the present study of turtle shell development involve the mechanism by which the carapacial ridge initiates shell formation and the mechanism by which each rib forms the costal bones adjacent to it. This paper claims that both sides of each debate might be correct-but within the species examined. Mechanism is more properly "mechanisms," and there is more than one single way to initiate carapace formation and to form the costal bones. In the initiation of the shell, the rib precursors may be kept dorsal by either "axial displacement" (in the hard-shell turtles) or "axial arrest" (in the soft-shell turtle Pelodiscus), or by a combination of these. The former process would deflect the rib into the dorsal dermis and allow it to continue its growth there, while the latter process would truncate rib growth. In both instances, though, the result is to keep the ribs from extending into the ventral body wall. Our recent work has shown that the properties of the carapacial ridge, a key evolutionary innovation of turtles, differ greatly between these two groups. Similarly, the mechanism of costal bone formation may differ between soft-shell and hard-shell turtles, in that the hard-shell species may have both periosteal flattening as well as dermal bone induction, while the soft-shelled turtles may have only the first of these processes.
目前龟壳发育研究中的两个主要争议涉及两个机制，一个是背甲嵴启动龟壳形成的机制，另一个是每根肋骨形成与其相邻的肋骨头的机制。本文认为，每场争论的双方可能都是正确的——但仅限于所研究的物种。机制更确切地说是“多种机制”，启动龟壳形成和形成肋骨头的方式不止一种。在龟壳形成的起始阶段，肋骨原基可能通过“轴向移位”（硬壳龟的情况）或“轴向停滞”（软壳龟中华鳖的情况），或两者结合的方式保持在背部。前一种过程会使肋骨偏向背部真皮并使其在那里继续生长，而后一种过程会截断肋骨的生长。不过，在这两种情况下，结果都是阻止肋骨延伸到腹侧体壁。我们最近的研究表明，背甲嵴这一龟类关键的进化创新特征在这两类龟之间有很大差异。同样，软壳龟和硬壳龟形成肋骨头的机制可能也有所不同，硬壳龟类可能既有骨膜扁平化又有真皮骨诱导，而软壳龟可能只有前一种过程。
