男性雖然強(qiáng)悍,，他們的Y染色體卻是羸弱的。近年來,，一些科學(xué)家甚至認(rèn)為Y染色體正呈萎縮之勢,，預(yù)計(jì)會(huì)在500萬年內(nèi)消失，而男性也會(huì)隨之滅絕,。不過，近日美國科學(xué)家的一項(xiàng)研究顯示,，上述觀點(diǎn)實(shí)在是“危言聳聽”,，這個(gè)世界上還會(huì)一直存在男人的身影。相關(guān)論文發(fā)表在近期的Nature雜志上,。
染色體是基因的載體,。人類的細(xì)胞中有23對(duì)染色體，其中包括22對(duì)常染色體和1對(duì)性染色體,。性染色體包括X和Y染色體,，決定人類的性別。男性的性染色體為XY,，而女性為XX,。
與“勤勞強(qiáng)大”的X染色體相比，男性特有的Y染色體顯得“懶惰矮小”,。它不但體積不及X染色體的1/3,，而且其中絕大多數(shù)DNA序列都是沒有功能的，參與“工作”的基因甚至不及X染色體的1/10,。
但是,，Y染色體也有自己的核心競爭力，它的存在對(duì)人類性別的決定具有重要意義,，其王牌就是SRY基因,。SRY基因，又稱睪丸決定因子,，在胚胎的性分化過程中,，SRY基因刺激男性胚胎完全向男性化的方向發(fā)展,。可以這樣說,，沒有SRY基因,，世界上就沒有男人。
幾年前,，在一次名為“Y染色體的衰退和消亡以及男性未來”的講座中,，澳大利亞國立大學(xué)基因研究專家珍妮弗·格雷夫斯指出：“3億年前每個(gè)Y染色體上大約有1400個(gè)基因，而現(xiàn)在只剩下45個(gè),。按照這種衰減速度,，500萬年后Y染色體上就不再有任何基因了。Y染色體正在消亡,，男性群體可能最終滅絕,。”
格雷夫斯的觀點(diǎn)經(jīng)媒體報(bào)道后，在社會(huì)上引起極大反響,。為了確定“Y染色體消亡說”是否存在夸大事實(shí)的嫌疑,，最近美國懷海德生物醫(yī)學(xué)研究所研究人員珍妮弗·休斯等人在比較了人和恒河猴的Y染色體上的基因后發(fā)現(xiàn)，人類Y染色體上基因衰減的速度正逐漸降低,，幾乎進(jìn)入停滯狀態(tài),。
人和恒河猴曾經(jīng)擁有共同的祖先，在他們走上各自進(jìn)化道路的2500萬年里,，雖然人類Y染色體的長度遠(yuǎn)遠(yuǎn)超過恒河猴,，但是他們Y染色體的基因構(gòu)成基本一致。
休斯說,，與恒河猴Y染色體相比,，人類Y染色體2500萬年來只流失了一個(gè)基因，而在過去600萬年里,，人類Y染色體上的基因流失數(shù)為零,，其基因衰減的速度越來越慢。“所以,，我相信即使再過5000萬年,，人類Y染色體依然會(huì)存在，‘Y染色體消亡說’可以就此打住了,。”（生物谷Bioon.com）
doi:10.1038/nature10843
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Strict evolutionary conservation followed rapid gene loss on human and rhesus Y chromosomes
Jennifer F. Hughes, Helen Skaletsky, Laura G. Brown, Tatyana Pyntikova, Tina Graves, Robert S. Fulton, Shannon Dugan, Yan Ding, Christian J. Buhay, Colin Kremitzki, Qiaoyan Wang, Hua Shen, Michael Holder, Donna Villasana, Lynne V. Nazareth, Andrew Cree, Laura Courtney, Joelle Veizer, Holland Kotkiewicz, Ting-Jan Cho, Natalia Koutseva, Steve Rozen, Donna M. Muzny, Wesley C. Warren, Richard A. Gibbs
The human X and Y chromosomes evolved from an ordinary pair of autosomes during the past 200–300 million years. The human MSY (male-specific region of Y chromosome) retains only three percent of the ancestral autosomes’ genes owing to genetic decay. This evolutionary decay was driven by a series of five ‘stratification’ events. Each event suppressed X–Y crossing over within a chromosome segment or ‘stratum’, incorporated that segment into the MSY and subjected its genes to the erosive forces that attend the absence of crossing over. The last of these events occurred 30 million years ago, 5 million years before the human and Old World monkey lineages diverged. Although speculation abounds regarding ongoing decay and looming extinction of the human Y chromosomeremarkably little is known about how many MSY genes were lost in the human lineage in the 25 million years that have followed its separation from the Old World monkey lineage. To investigate this question, we sequenced the MSY of the rhesus macaque, an Old World monkey, and compared it to the human MSY. We discovered that during the last 25 million years MSY gene loss in the human lineage was limited to the youngest stratum (stratum 5), which comprises three percent of the human MSY. In the older strata, which collectively comprise the bulk of the human MSY, gene loss evidently ceased more than 25 million years ago. Likewise, the rhesus MSY has not lost any older genes (from strata 1–4) during the past 25 million years, despite its major structural differences to the human MSY. The rhesus MSY is simpler, with few amplified gene families or palindromes that might enable intrachromosomal recombination and repair. We present an empirical reconstruction of human MSY evolution in which each stratum transitioned from rapid, exponential loss of ancestral genes to strict conservation through purifying selection.
