生物谷：約翰霍普金斯大學的研究人員獲得的新證據(jù)再一次證明許多通過進化產(chǎn)生的遺傳小片段對人類或其他動物不造成任何優(yōu)勢或劣勢。
很長一段時間,，進化的基礎理論認為所有試圖保留下來的隨機遺傳變化都具有某種選擇優(yōu)勢,。這項新的研究進一步證實我們之所以是今天這個樣子主要歸因于完全中立的隨機突變。
霍普金斯的副教授Nicholas Katsanis表示,，他不是完全貶低自然選擇（帶來某種優(yōu)勢的持續(xù)性遺傳變化）的作用,，這個過程還是很有價值的。這項新研究只是再次證明和強調(diào)了與自然選擇等同甚至是更強大的中立遺傳物質(zhì)的重要性,。
研究人員將這項研究的結(jié)果公布在近期的《科學公共圖書館·遺傳學》（PLoS Genetics）網(wǎng)絡版上,。霍普金斯的實驗證實,，人類基因組的一種主要的結(jié)構(gòu)標志物——DNA重復元件（這些成分占到我們基因組的40%多）的顯著特征就是對它所棲息的生物體不提供任何的益處,。重復元件是包含相同重復序列堿基對的DNA片段。
Katsanis和他的研究組在分析一種罕見疾病——Bardet Biedl綜合癥的相關(guān)基因時,，一種類型的重復元件讓他們困惑不已,。在尋找新基因時,，他們發(fā)現(xiàn)了從線粒體拷貝得來的DNA成分。這些線粒體遺傳序列被稱為線粒體假基因（Numts）,。細胞核線粒體假基因（Numts）是線粒體DNA轉(zhuǎn)移到核DNA中的片段,，之前的研究顯示其對系統(tǒng)發(fā)育、群體遺傳和細胞核突變模式等研究有重要作用,。（Bardet-Biedl syndrome（巴德-畢德氏癥候群）（簡稱BBS）是一種體染色體隱性遺傳疾病,，病患在出生后有肥胖問題，有多指/趾畸形（polydactyly）,，視網(wǎng)膜漸漸的萎縮,，性腺發(fā)育不全，腎臟畸形及學習困難,。早在1920年Bardet和Biedl首先描述這些癥狀,，其后有超過三百個案例的報告,。）
當他們將研究的范圍擴展到整個人類基因組時,，他們發(fā)現(xiàn)了超過1200個不同長度的線粒體DNA片段被包埋在染色體中。盡管黑猩猩具有相當數(shù)量的這種成分,，但小鼠和大鼠則只有大約600個假基因,。由于這種假基因數(shù)量隨著物種的等級升高而增加，因此這可能意味著存在某種進化上的目的而保留它們,。
但是,，令人驚訝的是，沒有任何一個假基因被包含在真基因中來制造一種起某種功能的蛋白質(zhì),，而且它們似乎也不控制附近任何基因的功能,。這些分析結(jié)果顯示，它們至多是我們基因組中的一個中立部分,。
原始出處:
PLoS Genetics
Received: February 13, 2007; Accepted: June 4, 2007; Published: July 20, 2007
Population Bottlenecks as a Potential Major Shaping Force of Human Genome Architecture
Adrian Gherman1, Peter E. Chen1, Tanya M. Teslovich1, Pawel Stankiewicz2, Marjorie Withers2, Carl S. Kashuk1, Aravinda Chakravarti1, James R. Lupski2,3,4, David J. Cutler1*, Nicholas Katsanis1,5*
1 McKusick-Nathans Institute of Genetic Medicine, Johns Hopkins University, Baltimore, Maryland, United States of America, 2 Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, Texas, United States of America, 3 Department of Pediatrics, Baylor College of Medicine, Houston, Texas, United States of America, 4 Texas Children's Hospital, Houston, Texas, United States of America, 5 Wilmer Eye Institute, Johns Hopkins University, Baltimore, Maryland, United States of America
The modern synthetic view of human evolution proposes that the fixation of novel mutations is driven by the balance among selective advantage, selective disadvantage, and genetic drift. When considering the global architecture of the human genome, the same model can be applied to understanding the rapid acquisition and proliferation of exogenous DNA. To explore the evolutionary forces that might have morphed human genome architecture, we investigated the origin, composition, and functional potential of numts (nuclear mitochondrial pseudogenes), partial copies of the mitochondrial genome found abundantly in chromosomal DNA. Our data indicate that these elements are unlikely to be advantageous, since they possess no gross positional, transcriptional, or translational features that might indicate beneficial functionality subsequent to integration. Using sequence analysis and fossil dating, we also show a probable burst of integration of numts in the primate lineage that centers on the prosimian–anthropoid split, mimics closely the temporal distribution of Alu and processed pseudogene acquisition, and coincides with the major climatic change at the Paleocene–Eocene boundary. We therefore propose a model according to which the gross architecture and repeat distribution of the human genome can be largely accounted for by a population bottleneck early in the anthropoid lineage and subsequent effectively neutral fixation of repetitive DNA, rather than positive selection or unusual insertion pressures.
