Patterning and post-patterning modes of evolutionary digit loss in mammals

摘要

四足动物四肢的基本五趾结构在演化过程中以很多方式被改变过很多次,通常是通过趾头的渐进性消失被改变的。本期Nature上发表的两篇论文研究了造成哺乳动物趾头减少的发育变化。Javier Lopez-Rios等人研究的对象是牛,它们的第三趾和第四趾被改变形成了蹄子,第二趾和第五趾只留下了残迹,第一趾消失了。第一个"肢芽"被发现是在发育过程中渐进性消失的。编码"肢发育形态发生素"Sonic hedgehog (SHH)的一个受体的Ptch1基因,因一个Ptch1顺式调控模块(该模块在牛handplate 发育过程中不再对分级的SHH信号传导做出反应）的演化改变而被上调。Kimberly Cooper等人利用一系列不同的哺乳动物发现,"趾头消失"（digit loss)的机制在不同类型的动物中是不同的。在像跳鼠和骆驼这样相差如此之大的动物中,细胞死亡让新出现的腿中的组织离开剩余的趾头。在其他动物（如猪）中,"趾头消失"是由早先的腿(肢)模式形成事件协调的,细胞死亡数量并不增加。综合起来,这些发现展现了有蹄动物和啮齿类动物腿（肢）演化机制的显著可塑性,但也显示了一定程度的演化趋同。%A reduction in the number of digits has evolved many times in tetrapods, particularly in cursorial mammals that travel over deserts and plains, yet the underlying developmental mechanisms have remained elusive. Here we show that digit loss can occur both during early limb patterning and at later post-patterning stages of chondrogenesis. In the 'odd-toed' jerboa (Dipus sagitta) and horse and the 'even-toed' camel, extensive cell death sculpts the tissue around the remaining toes. In contrast, digit loss in the pig is orchestrated by earlier limb patterning mechanisms including downregulation of Ptch1 expression but no increase in cell death. Together these data demonstrate remarkable plasticity in the mechanisms of vertebrate limb evolution and shed light on the complexity of morphological convergence, particularly within the artiodactyl lineage.