Cordero Gerardo Antonio, Quinteros Kevin
Department of Ecology, Evolution, and Organismal Biology, Iowa State University, 251 Bessey Hall, Ames, IA 50011, USA
Department of Ecology, Evolution, and Organismal Biology, Iowa State University, 251 Bessey Hall, Ames, IA 50011, USA.
Biol Lett. 2015 Apr;11(4):20150022. doi: 10.1098/rsbl.2015.0022.
Recent efforts to decipher the enigma of the turtle's shell revealed that distantly related turtle species deploy diverse processes during shell development. Even so, extant species share in common a shoulder blade (scapula) that is encapsulated within the shell. Thus, evolutionary change in the correlated development of the shell and scapula probably underpins the evolution of highly derived shell morphologies. To address this expectation, we conducted one of the most phylogenetically comprehensive surveys of turtle development, focusing on scapula growth and differentiation in embryos, hatchlings and adults of 13 species. We report, to our knowledge, the first description of secondary differentiation owing to skeletal remodelling of the tetrapod scapula in turtles with the most structurally derived shell phenotypes. Remodelling and secondary differentiation late in embryogenesis of box turtles (Emys and Terrapene) yielded a novel skeletal segment (i.e. the suprascapula) of high functional value to their complex shell-closing system. Remarkably, our analyses suggest that, in soft-shelled turtles (Trionychidae) with extremely flattened shells, a similar transformation is linked to truncated scapula growth. Skeletal remodelling, as a form of developmental plasticity, might enable the seemingly constrained turtle body plan to diversify, suggesting the shell is not an evolutionary straitjacket.
最近，人们为解开龟壳之谜所做的努力表明，亲缘关系较远的龟类物种在龟壳发育过程中采用了不同的过程。即便如此，现存物种都有一个共同特征，即肩胛骨被包裹在龟壳内。因此，龟壳和肩胛骨相关发育过程中的进化变化可能是高度特化龟壳形态进化的基础。为了验证这一推测，我们开展了一项龟类发育的系统发育综合调查，重点研究了13个物种的胚胎、幼体和成体的肩胛骨生长与分化情况。据我们所知，我们首次描述了具有最复杂结构的龟壳表型的龟类中，四足动物肩胛骨骨骼重塑导致的二次分化现象。盒龟（潮龟属和锦龟属）胚胎发育后期的重塑和二次分化产生了一个新的骨骼节段（即上肩胛骨），对其复杂的龟壳闭合系统具有很高的功能价值。值得注意的是，我们的分析表明，在龟壳极度扁平的软壳龟（鳖科）中，类似的转变与肩胛骨生长截断有关。骨骼重塑作为一种发育可塑性的形式，可能使看似受限的龟类身体结构多样化，这表明龟壳并非进化的束缚。
