进化生物学研究面临的一个最大挑战就是破译复杂结构的起源。如今，科学家终于揭开了细菌鞭毛——一种像尾巴一样的微小结构，用于游水以及侵入宿主——进化的步骤。一项新的研究表明，鞭毛是细菌祖先的一个特定基因连续复制的结果。这一发现不但回答了关于复杂结构进化的一个重要问题，同时为对付进化论反对者的对立观点提供了新的“弹药”。
大约有50多个基因与鞭毛的构造及功能有关。对于鞭毛起源，科学家曾提出多种假设，但是没有一种理论能够在遗传水平上对这种细胞器官如何出现给出充分的说明。为了搞清这一问题，美国图森市亚利桑那大学的进化生物学家Howard Ochman与博士后Renyi Liu在获取了41种生有鞭毛的细菌的全部基因组后，对所有微生物中与鞭毛相关的24种基因进行了研究。
在每一种细菌中，这24种基因彼此都非常类似，但是它们与基因组中的其他基因却没有相同之处。考虑到这些基因均存在于所有长鞭毛的细菌中，Ochman认为，该发现意味着这一系列基因源于这些细菌共有祖先的一个特定基因的复制。基因的细微变化会产生新的功能。每个基因都与一个不同的形态特征有关，例如产生蛋白质从而形成鞭毛的运动神经、丝状体和其他结构成分。此外，研究人员创建的一棵进化树表明，这些基因出现的位置与鞭毛演化步骤的顺序相互一致。Ochman和Liu最近在美国《国家科学院院刊》网络版上报告了这一研究成果。
布卢明顿市印第安纳大学的进化生物学家Michael Lynch表示，这一发现突出了生物进化的几个重要原则。他说：“复杂结构是建立在简单结构基础之上的，而这项研究成果正是上述理论的最好证明。”布朗大学的细胞生物学家Ken Miller同时指出，对于那些“智能设计”的支持者而言，这项研究提供了一个有力的反例，表明鞭毛能够从一个简单基因进化而来。Miller说：“通过在许多生有鞭毛细菌的共同祖先中验证这一假设，研究人员清楚地证明了这些基因源自另一个基因的复制过程。”
原始出处:
Published online before print April 16, 2007, 10.1073/pnas.0700266104
PNAS | April 24, 2007 | vol. 104 | no. 17 | 7116-7121
OPEN ACCESS ARTICLE
Stepwise formation of the bacterial flagellar system
Renyi Liu*, and Howard Ochman*,,
Departments of *Biochemistry and Molecular Biophysics and Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721
Edited by Francisco J. Ayala, University of California, Irvine, CA, and approved March 8, 2007 (received for review January 11, 2007)
Elucidating the origins of complex biological structures has been one of the major challenges of evolutionary studies. The bacterial flagellum is a primary example of a complex apparatus whose origins and evolutionary history have proven difficult to reconstruct. The gene clusters encoding the components of the flagellum can include >50 genes, but these clusters vary greatly in their numbers and contents among bacterial phyla. To investigate how this diversity arose, we identified all homologs of all flagellar proteins encoded in the complete genome sequences of 41 flagellated species from 11 bacterial phyla. Based on the phylogenetic occurrence and histories of each of these proteins, we could distinguish an ancient core set of 24 structural genes that were present in the common ancestor to all Bacteria. Within a genome, many of these core genes show sequence similarity only to other flagellar core genes, indicating that they were derived from one another, and the relationships among these genes suggest the probable order in which the structural components of the bacterial flagellum arose. These results show that core components of the bacterial flagellum originated through the successive duplication and modification of a few, or perhaps even a single, precursor gene.
bacterial evolution | biological complexity | gene duplication
