生物谷报道：地球上最早的四足动物出现在3.65亿年前，通过对斑马鱼（zebrafish）和四足动物体内一种名为Hox的基因进行研究，科学家长期以来认为，原始的鱼类不具有与四肢形成相关的Hox基因的另一个表达阶段，因而推断四肢的进化是由于遗传变异形成了全新的基因功能。
然而，科学家的一项最新研究颠覆了这一传统的理论。比斑马鱼更加原始的鱼类就已经具有与四肢分化相关的基因表达方式，四肢的出现并不是一个全新的进化事件。这一成果发表在5月24日的《自然》杂志上。
进行此项研究的是美国芝加哥大学的Neil Shubin、Marcus Davis和Randall Dahn等，他们在一种十分古老的鱼类——匙吻鲟（paddlefish）体内找到了上述的另一个表达阶段，证实了形成胸鳍与形成四肢的Hox基因活动十分相似。该发现同时表明，这种脊椎动物特有的基因表达方式实际上更为古老。
在此之前，Shubin等科学家发现并研究了一种3.75亿年前的物种Tiktaalik roseae，从形态学上填补了鱼类与陆生动物之间的进化缺失。现在科学家无疑又从遗传层面上揭示了四肢的演化。
不过，Shubin表示，尽管这种基因表达方式肯定在现代四肢动物的四肢进化过程中起到了重要作用，但是在原始鱼类中，这可能并不是它们的本意。也就是说，形成带手脚趾的四肢的这种能力一直存在着，但在一系列的环境刺激下，这种能力才最终得到发展。
Shubin实验室目前已经开始对鲨鱼进行研究，看类似的基因表达是否存在于这种更古老的物种的进化过程中。（科学网 任霄鹏/编译）
原始出处：
Nature 447, 473-476 (24 May 2007) | doi:10.1038/nature05838; Received 26 February 2007; Accepted 13 April 2007
An autopodial-like pattern of Hox expression in the fins of a basal actinopterygian fish
Marcus C. Davis1, Randall D. Dahn1 & Neil H. Shubin1,2
Department of Organismal Biology and Anatomy, The University of Chicago, Chicago, Illinois 60637, USA
Field Museum of Natural History, 1400 South Lake Shore Drive, Chicago, Illinois 60605, USA
Correspondence to: Neil H. Shubin1,2 Correspondence and requests for materials should be addressed to N.H.S. (Email: nshubin@uchicago.edu).
Top of page
Comparative analyses of Hox gene expression and regulation in teleost fish and tetrapods support the long-entrenched notion that the distal region of tetrapod limbs, containing the wrist, ankle and digits, is an evolutionary novelty1, 2, 3, 4. Data from fossils support the notion that the unique features of tetrapod limbs were assembled over evolutionary time in the paired fins of fish5. The challenge in linking developmental and palaeontological approaches has been that developmental data for fins and limbs compare only highly derived teleosts and tetrapods; what is lacking are data from extant taxa that retain greater portions of the fin skeletal morphology considered primitive to all bony fish6, 7. Here, we report on the expression and function of genes implicated in the origin of the autopod in a basal actinopterygian, Polyodon spathula. Polyodon exhibits a late-phase, inverted collinear expression of 5' HoxD genes, a pattern of expression long considered a developmental hallmark of the autopod and shown in tetrapods to be controlled by a 'digit enhancer' region. These data show that aspects of the development of the autopod are primitive to tetrapods and that the origin of digits entailed the redeployment of ancient patterns of gene activity.
