2012年8月12日 訊 /生物谷BIOON/ --8月6日,，刊登在國際著名雜志PNAS上的兩篇研究報告通過重點(diǎn)研究番木瓜（papaya）來揭示性染色體的進(jìn)化。研究者表示,，番木瓜的性染色體在短期的進(jìn)化過程中發(fā)生了劇烈的改變,，人類性染色體進(jìn)化了超過1億6千萬年,，而番木瓜則只進(jìn)化了700萬年,。研究者的其中一項(xiàng)研究對比了番木瓜的性染色體和常染色體，另一項(xiàng)研究則對比了X染色體和Y染色體的差別,。
研究發(fā)現(xiàn),，番木瓜的性染色體主要通過積累重復(fù)序列的方式來不斷增加其尺寸，于此同時重新自我組裝,，缺失某些基因,。而起Y染色體獨(dú)立地從常染色體獲得基因，Y染色體基因的缺失是有記錄的,，但是X染色體基因的缺失,，尤其是在早期，卻是意料不到的,。相比X染色體,，Y染色體中獲得重復(fù)序列和缺失基因的速度更快一些,。
研究者Andrea表示，這是我們首次揭示性染色體早期階段的進(jìn)化,，通過人們會關(guān)注古老的性染色體,，因?yàn)槠浜臀覀兏酉嚓P(guān)一些。分析X染色體對于理解性的進(jìn)化非常重要,，在番木瓜中的新發(fā)現(xiàn)也揭示了人類X染色體發(fā)生的巨大變化,，這種改變并不會被檢測到，因?yàn)檫z傳的常染色體作為對照不在可用,。
由于番木瓜的性染色體比較年輕,，可以與常染色體進(jìn)行對比，因此其可以給出一些關(guān)于X和Y染色體進(jìn)化的一些早期事件,。研究番木瓜的染色體是一項(xiàng)非常艱巨的任務(wù),，番木瓜有雌性、雄性和雌雄同體三種性型,。因?yàn)槠鋸?fù)雜性促使了番木瓜種植者的很多問題,，雌雄同體是番木瓜最具有生產(chǎn)力的性型，而且可以產(chǎn)出最好的水果,，但是其后代卻不是雌雄同體的,。
當(dāng)研究者對比了X染色體和雌雄同體的Yh染色體（其Y染色體發(fā)生了輕微改變）發(fā)現(xiàn)，Yh性別決定區(qū)域的兩種主要序列發(fā)生了倒位,。第一種發(fā)生在700萬年以前,，另外一種發(fā)生在190萬年以前，因此會造成后來出現(xiàn)的分化,。
研究者的研究發(fā)現(xiàn)改變了我們對于X染色體進(jìn)化的視角,，尤其是X染色體的進(jìn)化。如今我們知道X染色體和Y染色體在進(jìn)化早期階段發(fā)揮著巨大的變化,。并不僅僅是Y染色體發(fā)生著變化,。（生物谷Bioon.com）
編譯自：Examining the early evolution of sex chromosomes
doi:10.1073/pnas.1207833109
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PMID:
Sequencing papaya X and Yh chromosomes reveals molecular basis of incipient sex chromosome evolution
Jianping Wanga,1,2, Jong-Kuk Naa,1,3, Qingyi Yub,c,1, Andrea R. Gschwenda,1, Jennifer Hana, Fanchang Zenga, Rishi Aryala, Robert VanBurena, Jan E. Murraya, Wenli Zhangd, Rafael Navajas-Péreze,4, F. Alex Feltuse,5, Cornelia Lemkee, Eric J. Tongc, Cuixia Chena,6, Ching Man Waic,f, Ratnesh Singhc, Ming-Li Wangc, Xiang Jia Ming, Maqsudul Alamh, Deborah Charlesworthi, Paul H. Moorec, Jiming Jiangd, Andrew H. Patersone, and Ray Minga,7
Sex determination in papaya is controlled by a recently evolved XY chromosome pair, with two slightly different Y chromosomes controlling the development of males (Y) and hermaphrodites (Yh). To study the events of early sex chromosome evolution, we sequenced the hermaphrodite-specific region of the Yh chromosome (HSY) and its X counterpart, yielding an 8.1-megabase (Mb) HSY pseudomolecule, and a 3.5-Mb sequence for the corresponding X region. The HSY is larger than the X region, mostly due to retrotransposon insertions. The papaya HSY differs from the X region by two large-scale inversions, the first of which likely caused the recombination suppression between the X and Yh chromosomes, followed by numerous additional chromosomal rearrangements. Altogether, including the X and/or HSY regions, 124 transcription units were annotated, including 50 functional pairs present in both the X and HSY. Ten HSY genes had functional homologs elsewhere in the papaya autosomal regions, suggesting movement of genes onto the HSY, whereas the X region had none. Sequence divergence between 70 transcripts shared by the X and HSY revealed two evolutionary strata in the X chromosome, corresponding to the two inversions on the HSY, the older of which evolved about 7.0 million years ago. Gene content differences between the HSY and X are greatest in the older stratum, whereas the gene content and order of the collinear regions are identical. Our findings support theoretical models of early sex chromosome evolution.
