進化生物學研究面臨的一個最大挑戰(zhàn)就是破譯復雜結構的起源,。如今,，科學家終于揭開了細菌鞭毛——一種像尾巴一樣的微小結構,，用于游水以及侵入宿主——進化的步驟。一項新的研究表明,，鞭毛是細菌祖先的一個特定基因連續(xù)復制的結果,。這一發(fā)現(xiàn)不但回答了關于復雜結構進化的一個重要問題，同時為對付進化論反對者的對立觀點提供了新的“彈藥”,。
大約有50多個基因與鞭毛的構造及功能有關,。對于鞭毛起源，科學家曾提出多種假設,，但是沒有一種理論能夠在遺傳水平上對這種細胞器官如何出現(xiàn)給出充分的說明,。為了搞清這一問題，美國圖森市亞利桑那大學的進化生物學家Howard Ochman與博士后Renyi Liu在獲取了41種生有鞭毛的細菌的全部基因組后,，對所有微生物中與鞭毛相關的24種基因進行了研究,。
在每一種細菌中，這24種基因彼此都非常類似,，但是它們與基因組中的其他基因卻沒有相同之處,。考慮到這些基因均存在于所有長鞭毛的細菌中,，Ochman認為,，該發(fā)現(xiàn)意味著這一系列基因源于這些細菌共有祖先的一個特定基因的復制?；虻募毼⒆兓瘯a(chǎn)生新的功能,。每個基因都與一個不同的形態(tài)特征有關,，例如產(chǎn)生蛋白質(zhì)從而形成鞭毛的運動神經(jīng)、絲狀體和其他結構成分,。此外,，研究人員創(chuàng)建的一棵進化樹表明，這些基因出現(xiàn)的位置與鞭毛演化步驟的順序相互一致,。Ochman和Liu最近在美國《國家科學院院刊》網(wǎng)絡版上報告了這一研究成果,。
布盧明頓市印第安納大學的進化生物學家Michael Lynch表示，這一發(fā)現(xiàn)突出了生物進化的幾個重要原則,。他說：“復雜結構是建立在簡單結構基礎之上的,，而這項研究成果正是上述理論的最好證明。”布朗大學的細胞生物學家Ken Miller同時指出,，對于那些“智能設計”的支持者而言,，這項研究提供了一個有力的反例，表明鞭毛能夠從一個簡單基因進化而來,。Miller說：“通過在許多生有鞭毛細菌的共同祖先中驗證這一假設,，研究人員清楚地證明了這些基因源自另一個基因的復制過程。”
原始出處:
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
