大约5000万年前，鲸类的祖先用四条腿从陆地走向海洋，为适应海洋生存环境，其后肢不断退化并几乎消失，而前肢却进化成鳍状肢。鲸类鳍状肢为何会出现？是不是基因变异的结果？华东师大发布信息：由校生命科学学院的博士后王及其合作导师，以及英国合作者的最新研究成果为解答这一难题提供了重要线索。
据生物学家研究，绝大多数动物的身体器官都受一个名为Hox的基因家族控制。而动物的前肢发育特别受到Hoxd12和Hoxd13基因的控制。这两个基因的突变会导致动物前肢的畸形。王等研究人员通过对鲸类和其它哺乳动物类群的Hoxd12和Hoxd13基因测序，发现了这两个基因在鲸类鳍状肢的起源与分化中起到了重要作用。即鲸和现存的河马、牛、猪等偶蹄目动物具有同一祖先，前肢都有4个独立的指头，但是由于这两个基因的突变，鲸类祖先的前肢多长出了1个指头，并且指间长出了蹼。在其后鲸类的再次进化过程中，部分须鲸的前肢却又从5指进化成4指，恢复了进化前的指头数量。
研究人员还发现这两个Hox基因在鲸类的平均进化速率，均显著高于其它哺乳动物类群，并最终认定，Hox基因的适应性进化时间与鲸类鳍状肢的宏观进化时代完全相符，是自然选择的结果，而非偶然形成的。华东师大研究人员的相关研究论文《5’HoxD基因的适应性进化在鲸类鳍状肢的起源与分化中的作用》，于近日发表在国际权威杂志《分子生物学与进化》（Molecular Biology and Evolution）上。(生物谷Bioon.com)
生物谷推荐原始出处：
Molecular Biology and Evolution, doi:10.1093/molbev/msn282
Adaptive evolution of 5’HoxD genes in the origin and diversification of the cetacean flipper
Zhe Wang*, Lihong Yuan, Stephen J. Rossiter, Xueguo Zuo*, Binghua Ru*, Hui Zhong*, Naijian Han, Gareth Jones?, Paul D. Jepson and Shuyi Zhang*
* School of Life Science, East China Normal University, Shanghai, China
Guangdong Entomological Institute, Guangzhou, China
School of Biological and Chemical Sciences, Queen Mary, University of London, London, UK
Institute of Zoology, Chinese Academy of Sciences, Beijing, China
School of Biological Sciences, University of Bristol, Bristol, UK
Institute of Zoology, Zoological Society of London, Regent's Park, London, UK
The homeobox genes Hoxd12 and Hoxd13 control digit patterning and limb formation in tetrapods. Both show strong expression in the limb bud during embryonic development, are highly conserved across vertebrates, and show mutations that are associated with carpal, metacarpal and phalangeal deformities. The most dramatic evolutionary reorganization of the mammalian limb has occurred in cetaceans (whales, dolphins and porpoises), in which the hindlimbs have been lost and the forelimbs have evolved into paddle-shaped flippers. We reconstructed the phylogeny of digit patterning in mammals, and inferred that digit number has changed twice in the evolution of the cetacean forelimb. First, the divergence of the early cetaceans from their even-toed relatives coincided with the reacquisition of the pentadactyl forelimb, whereas the ancestors of tetradactyl baleen whales (Mysticeti) later lost a digit again. To test whether the evolution of the cetacean forelimb is associated with positive selection or relaxation of Hoxd12 and Hoxd13, we sequenced these genes in a wide range of mammals. In Hoxd12, we found evidence of Darwinian selection associated with both episodes of cetacean forelimb reorganization. In Hoxd13, we found a novel expansion of a polyalanine tract in cetaceans compared to other mammals (17/18 residues versus 14/15 residues, respectively), lengthening of which has previously been shown to be linked to synpolydactyly in humans and mice. Both genes also show much greater sequence variation among cetaceans than across other mammalian lineages. Our results strongly implicate 5’HoxD genes in the modulation of digit number, web forming, and the high morphological diversity of the cetacean manus.
