Shapiro Michael D, Hanken James, Rosenthal Nadia
Department of Organismic and Evolutionary Biology, Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138, USA.
J Exp Zool B Mol Dev Evol. 2003 Jun 15;297(1):48-56. doi: 10.1002/jez.b.19.
Loss of limb skeletal elements is a recurring theme in tetrapod evolution, but the developmental mechanisms underlying this phenomenon remain largely unknown. The Australian lizard genus Hemiergis offers an excellent model system to study limb reduction among closely related, naturally occurring populations with different numbers of digits. Evolutionary digit loss in Hemiergis does not result from simple truncation of a pentadactyl skeletal developmental program. Rather, the duration of embryonic expression of the patterning molecule Sonic hedgehog (SHH) is shortened in limbs with reduced numbers of digits, and is correlated with decreased cell proliferation in the posterior aspect of the limb. Moreover, this comparative analysis suggests an early role for SHH in specification of digit identity and later importance in maintaining cell proliferation and survival. Subtle changes in spatial or temporal regulation of SHH may alter proliferation and patterning of the developing limb, thereby effecting divergence in adult limb morphology among closely related species. In contrast, expression of MSX and Distal-less proteins were similar among embryos from different populations.
四肢骨骼元素的缺失是四足动物进化中反复出现的主题，但这种现象背后的发育机制在很大程度上仍然未知。澳大利亚蜥蜴属Hemiergis提供了一个优秀的模型系统，用于研究亲缘关系密切、自然存在的不同指（趾）数种群中的肢体简化现象。Hemiergis中进化性的指（趾）缺失并非源于五趾型骨骼发育程序的简单截断。相反，在指（趾）数减少的肢体中，模式分子音猬因子（SHH）的胚胎表达持续时间缩短，并且与肢体后部细胞增殖的减少相关。此外，这种比较分析表明，SHH在指（趾）身份的特化中起早期作用，而在维持细胞增殖和存活方面起后期重要作用。SHH在空间或时间调控上的细微变化可能会改变发育中肢体的增殖和模式形成，从而导致亲缘关系密切的物种在成年肢体形态上出现差异。相比之下，来自不同种群的胚胎中MSX和远端缺失蛋白的表达相似。
