在長(zhǎng)達(dá)30多億年的生命進(jìn)化史上,，生命何時(shí)出現(xiàn)雌雄之分？這個(gè)問題一直困擾著科學(xué)家,，在2008年最后一期的世界著名雜志《基因組研究》上,，中外科學(xué)家發(fā)表了《番木瓜原始Y染色體的雄性特異區(qū)中DNA甲基化和異染色質(zhì)化》的研究論文，初步揭示了性染色體的起源變化機(jī)制,。番木瓜的雄性區(qū)域很有可能和人類2億至3億年前的Y染色體相似,。這種水果Y染色體基因?yàn)槿祟愋匀旧w起源和進(jìn)化的初始階段所發(fā)生的事件提供了間接證據(jù)。
論文作者之一,，南京農(nóng)業(yè)大學(xué)作物遺傳與種質(zhì)創(chuàng)新國(guó)家重點(diǎn)實(shí)驗(yàn)室王秀娥教授說,，番木瓜這種植物不同尋常，因?yàn)樗腥?mdash;—雄株,、雌株和雌雄同株,。代表它還正處于性別分化的起始階段。通過它或許能夠找到“性別起源”的秘密,。
專家在一株雌雄同株的番木瓜身上發(fā)現(xiàn)了一條剛剛處于起源階段的原始性染色體,，雄性區(qū)域很小，約占該條染色體的10%,，說明它是一條初始的性染色體,，同時(shí)這個(gè)雄性區(qū)域似乎已經(jīng)丟失了一些編碼蛋白的DNA。這種丟失通常被認(rèn)為是Y染色體從X染色體中分化的一個(gè)步驟,。專家說,，這樣的雄性區(qū)域與兩億年到三億年前人類的Y染色體的進(jìn)化狀況有相似之處。
專家們通過DNA分子原位雜交和免疫熒光分析直觀地研究了這條初始性染色體的重組抑制過程后發(fā)現(xiàn),，在番木瓜的雄性區(qū)域,，大約占到Y(jié)染色體的13%，而與X染色體相對(duì)應(yīng)的區(qū)域相比,，番木瓜的這段雄性區(qū)域積累了更多的DNA,，導(dǎo)致了該區(qū)域的X和Y染色體配對(duì)異常。而通過NDA的分子原位雜交,，專家在番木瓜的雄性區(qū)域發(fā)現(xiàn)了該區(qū)域有4個(gè)特異的異染色質(zhì)癤,，免疫熒光分析則揭示出這個(gè)區(qū)域與X染色體的相應(yīng)區(qū)域發(fā)生了較大的分化，具有高度的甲基化,。
這些研究結(jié)果為人類性染色體起源和進(jìn)化的初始階段所發(fā)生的事件,，提供了一個(gè)直接的證據(jù),。（生物谷Bioon.com）
生物谷推薦原始出處：
Genome Research Published in Advance July 1, 2008, doi:10.1101/gr.078808.108 Genome Res. 2008. 18: 1938-1943
DNA methylation and heterochromatinization in the male-specific region of the primitive Y chromosome of papaya
Wenli Zhang1,5, Xiue Wang1,2,5, Qingyi Yu3, Ray Ming4, and Jiming Jiang1,6
1 Department of Horticulture, University of Wisconsin, Madison, Wisconsin 53706, USA;
2 State Key Laboratory of Crop Genetics and Germplasm Enhancement, Nanjing Agricultural University, Nanjing, Jiangsu 210095, People’s Republic of China;
3 Hawaii Agriculture Research Center, Aiea, Hawaii 96701, USA;
4 Department of Plant Biology, University of Illinois at Urbana–Champaign, Urbana, Illinois 61801, USA
5 These authors contributed equally to this work.
Abstract
Sex chromosomes evolved from autosomes. Recombination suppression in the sex-determining region and accumulation of deleterious mutations lead to degeneration of the Y chromosomes in many species with heteromorphic X/Y chromosomes. However, how the recombination suppressed domain expands from the sex-determining locus to the entire Y chromosome remains elusive. The Y chromosome of papaya (Carica papaya) diverged from the X chromosome approximately 2–3 million years ago and represents one of the most recently emerged Y chromosomes. Here, we report that the male-specific region of the Y chromosome (MSY) spans ～13% of the papaya Y chromosome. Interestingly, the centromere of the Y chromosome is embedded in the MSY. The centromeric domain within the MSY has accumulated significantly more DNA than the corresponding X chromosomal domain, which leads to abnormal chromosome pairing. We observed four knob-like heterochromatin structures specific to the MSY. Fluorescence in situ hybridization and immunofluorescence assay revealed that the DNA sequences associated with the heterochromatic knobs are highly divergent and heavily methylated compared with the sequences in the corresponding X chromosomal domains. These results suggest that DNA methylation and heterochromatinization play an important role in the early stage of sex chromosome evolution.
