通常認為人類和黑猩猩之間僅有1%～2%的基因差異，但事實上,，區(qū)分人類和黑猩猩的基因比科學家預料的要多。一項新的研究表明,，把人類和近親——黑猩猩區(qū)分開的是人類獲得新基因,、拋棄舊基因的速率。
人類和黑猩猩這兩個物種500萬年以前還是一家,，通常認為現(xiàn)在只有1%～2%的基因差異,。但是這個百分比指的是基因中核糖的不同。然而,，進化不僅僅能夠修補基因序列,，即使基因本身不變，不同物種基因副本的數(shù)量也是可變的,?；蛴袝r增加，有時丟失,。然而,，定量計算這種得失很難，要求知道許多物種完整的基因組序列,。
A leg up.
The human species is turning over genes much faster than chimps and other mammals.
Credit: Michael K. Nichols/NGS/Getty Images
現(xiàn)在,，由于一些哺乳動物的基因組測序已經(jīng)完成，并有了一套新的統(tǒng)計方法,，美國印第安那大學的Matthew Hahn和同事進行了基因代謝（gene turnover）計算的研究,。他們測量了6種哺乳動物基因組中基因復制和丟失的速度。在觀察了1萬個基因家族中的大約12萬個基因后,，研究人員發(fā)現(xiàn)哺乳動物的基因代謝比犬類和嚙齒動物快,。而人類基因代謝的速率更快，是猴子的1.6倍,，非哺乳動物的2.8倍,。由于這么快速的淘汰更新，人類22000余個基因中,，有6.4%沒有在黑猩猩中表達出來,，這導致兩者表現(xiàn)出來的差別更加大了。
“你可以把基因組想象成旋轉(zhuǎn)門——基因通過它進進出出,。”Hahn的這一成果10月18號在線發(fā)表在《基因》上,。他表示,，基因代謝為自然選擇提供了“燃料”，迅速擴張的基因家族顯示出DNA的適應性變化,。其中一支異軍突起的基因家族是一組腦基因,，它們在人類中擴大了一倍多。
美國科羅拉多大學的基因組生物學家James Sikela表示,，該研究強調(diào)了“基因代謝在哺乳動物進化過程中的重要作用”,。但是他警告說，研究人員使用的最近“完成”的基因組序列可能會存在所謂的裝配誤差（assembly errors）,，并且很難證明一個基因完全缺失,，另外，基因多余的副本可能被忽視,。這些因素都會導致速率估計不準確,。不過Hahn表示，他們測試了誤差的影響,，在考慮誤差的情況下,，所得數(shù)據(jù)還是可以解釋人類快速的基因代謝率對進化的影響。
原始出處:
Genetics. Published Articles Ahead of Print: October 18, 2007, Copyright © 2007
doi:10.1534/genetics.107.080077
Accelerated rate of gene gain and loss in primates
Matthew W. Hahn 1*, Jeffery P. Demuth 1 and Sang-Gook Han 1
1 Indiana University
* To whom correspondence should be addressed. E-mail: [email protected] .
Submitted on August 6, 2007
Revised on September 4, 2007
Accepted on 4 September 2007
The molecular changes responsible for the evolution of modern humans have primarily been discussed in terms of individual nucleotide substitutions in regulatory or protein coding sequences. However, rates of nucleotide substitution are slowed in primates, and thus humans and chimpanzees are highly similar at the nucleotide level. We find that a third source of molecular evolution, gene gain and loss, is accelerated in primates relative to other mammals. Using a novel method that allows estimation of rate heterogeneity among lineages, we find that the rate of gene turnover in humans is more than 2.5X faster than in other mammals and may be due to both mutational and selective forces. By reconciling the gene trees for all of the gene families included in the analysis, we are able to independently verify the numbers of inferred duplications. We also use two methods based on the genome assembly of rhesus macaque to further verify our results. Our analyses identify several gene families that have expanded or contracted more rapidly than is expected even after accounting for an overall rate acceleration in primates, including brain-related families that have more than doubled in size in humans. Many of the families showing large expansions also show evidence for positive selection on their nucleotide sequences, suggesting that selection has been important in shaping copy-number differences among mammals. These findings may help explain why humans and chimpanzees show high similarity between orthologous nucleotides yet great morphological and behavioral differences.
Key Words: comparative genomics, duplication, gene family, positive selection, segmental duplication
