性別差異在鳥(niǎo)的世界中一直存在。一項(xiàng)新的研究表明,，與人類不同,，雄鳥(niǎo)比雌鳥(niǎo)獲得更多的特殊性別染色體后，鳥(niǎo)類并不會(huì)對(duì)此作出補(bǔ)償,。在此前提下,，雄鳥(niǎo)所表達(dá)的性別基因往往比雌鳥(niǎo)多40%——生物學(xué)家曾認(rèn)為這種不平衡現(xiàn)象可能是致命的。
提到X染色體,，就不得不說(shuō)說(shuō)人類的男性,。面對(duì)人類的X染色體和Y染色體，男性繼承了來(lái)自母親的兩個(gè)X染色體中的一半基因,。在這一過(guò)程中,，哺乳動(dòng)物雌性個(gè)體的兩條染色體有一條失活，以保持雌雄個(gè)體基因產(chǎn)物的劑量平衡,，這種現(xiàn)象就是所謂的劑量補(bǔ)償,。其他動(dòng)物，例如蒼蠅和線蟲(chóng)也采用了類似的策略,。而之前的研究表明,，鳥(niǎo)類——雄性具有兩個(gè)Z染色體,，雌性具有Z染色體和W染色體——可能并不是按照這一原則進(jìn)化的。但是導(dǎo)致這一結(jié)果的原因一直眾說(shuō)紛紜,、沒(méi)有定論,。
如今，來(lái)自美國(guó)加利福尼亞大學(xué)洛杉磯分校的一個(gè)研究小組通過(guò)一次偶然的機(jī)遇解決了這一謎題,。當(dāng)生理學(xué)家Arthur Arnold和同事取得這一研究成果時(shí),，他們正在分析鳴鳥(niǎo)兩性間大腦差異的遺傳基礎(chǔ)——與雌鳥(niǎo)相比，與Z染色體相關(guān)的基因在雄鳥(niǎo)中的表達(dá)要高出很多,。因此,，Arnold的研究小組決定繼續(xù)深入開(kāi)展研究。
利用名為微陣列的基因芯片,，研究人員比較了斑胸草雀,、雞、小鼠和人類的性別染色體基因表達(dá),。與預(yù)期的一樣,，哺乳動(dòng)物在X染色體基因的表達(dá)上并沒(méi)有太大的性別差異,。而與此相反,，雄斑胸草雀和公雞的Z染色體基因表達(dá)水平都比其在雌性中的表達(dá)高40%。研究小組在3月22日出版的《生物學(xué)雜志》（Journal of Biology）上報(bào)告了這一研究成果,。Arnold指出,，盡管如此，某些Z染色體基因還是在雄性與雌性之間表現(xiàn)出了較小的差異,，這意味著鳥(niǎo)類的劑量補(bǔ)償采取了一個(gè)較為適度的形式,。但是他說(shuō)，這可能是一種選擇的過(guò)程,，因?yàn)閮H僅影響“Z染色體基因很可能會(huì)造成大麻煩”,。
哥倫比亞市密蘇里大學(xué)的生物學(xué)家James Birchler表示，這一發(fā)現(xiàn)令人驚訝,，這是由于Z染色體由7%到10%的鳥(niǎo)類基因組構(gòu)成,。Birchler說(shuō)：“人們不禁會(huì)想，這樣大規(guī)模的沒(méi)有得到補(bǔ)償?shù)幕蚩赡苁怯泻Φ摹?rdquo;然而顯然在鳥(niǎo)類中并沒(méi)有出現(xiàn)這種情況,。他同時(shí)強(qiáng)調(diào),，由于ZW染色體系統(tǒng)的進(jìn)化早于XY染色體系統(tǒng)，因此這一研究結(jié)果對(duì)于搞清劑量補(bǔ)償?shù)倪M(jìn)化提供了重要線索,。
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Dosage compensation is less effective in birds than in mammals
Yuichiro Itoh* 1 , Esther Melamed* 1 , Xia Yang2 , Kathy Kampf1 , Susanna Wang2 , Nadir Yehya2 , Atila Van Nas2 , Kirstin Replogle3 , Mark R Band4 , David F Clayton3 , Eric E Schadt5 , Aldons J Lusis2 and Arthur P Arnold1
1Department of Physiological Science, University of California, Los Angeles, CA 90095, USA
2Department of Medicine, David Geffen School of Medicine, University of California, Los Angeles, CA 90095, USA
3Department of Cell and Developmental Biology, University of Illinois, Urbana, IL 61801, USA
4W.M. Keck Center for Comparative and Functional Genomics, University of Illinois, Urbana, IL 61801, USA
5Rosetta Inpharmatics, Seattle, WA 98034, USA
Journal of Biology 2007, 6:2 doi:10.1186/jbiol53
Abstract
Background
In animals with heteromorphic sex chromosomes, dosage compensation of sex-chromosome genes is thought to be critical for species survival. Diverse molecular mechanisms have evolved to effectively balance the expressed dose of X-linked genes between XX and XY animals, and to balance expression of X and autosomal genes. Dosage compensation is not understood in birds, in which females (ZW) and males (ZZ) differ in the number of Z chromosomes.
Results
Using microarray analysis, we compared the male:female ratio of expression of sets of Z-linked and autosomal genes in two bird species, zebra finch and chicken, and in two mammalian species, mouse and human. Male:female ratios of expression were significantly higher for Z genes than for autosomal genes in several finch and chicken tissues. In contrast, in mouse and human the male:female ratio of expression of X-linked genes is quite similar to that of autosomal genes, indicating effective dosage compensation even in humans, in which a significant percentage of genes escape X-inactivation.
Conclusion
Birds represent an unprecedented case in which genes on one sex chromosome are expressed on average at constitutively higher levels in one sex compared with the other. Sex-chromosome dosage compensation is surprisingly ineffective in birds, suggesting that some genomes can do without effective sex-specific sex-chromosome dosage compensation mechanisms.
