性染色體和B染色體（相對(duì)于正常染色體而言不遵循孟德爾遺傳分裂規(guī)律的染色體）的演化一直是經(jīng)典遺傳學(xué)長期未曾研究透徹的重要問題,。有意思的是,，在一種叫做銀額果蠅（Drosophila albomicans）的果蠅物種（如圖），新近演化出了非常年輕的性染色體和B染色體,，使其成為研究兩者的絕佳材料,。該果蠅僅有3對(duì)染色體,，其古老的性染色體和一對(duì)常染色體發(fā)生融合,，使得該對(duì)常染色體和性別連鎖,，相互之間無法重組,，成為擁有和古老性染色體同樣遺傳模式的‘新性染色體’(neo-sex chromosome),。另外，90年代初期王文研究員等人對(duì)該果蠅的B染色體研究發(fā)現(xiàn),，特定數(shù)目的B染色體將對(duì)銀額果蠅的后代產(chǎn)生顯著影響,，直接質(zhì)疑了一直以來B染色體完全只是‘自私遺傳因子’，沒有任何功能的傳統(tǒng)觀點(diǎn),。而這兩種系統(tǒng)的起源時(shí)間僅僅是12萬年,。
最近,，昆明動(dòng)物所已畢業(yè)博士生周琦和趙莉，在王文研究員指導(dǎo)下,，通過和深圳華大基因研究院和美國加州伯克利大學(xué)的合作,，對(duì)該果蠅進(jìn)行了全基因組的測定。他們運(yùn)用新一代Illumina測序技術(shù),，快速完成了該果蠅基因組的從頭組裝,，使它成為世界上第一個(gè)完全應(yīng)用新技術(shù)測序組裝的果蠅基因組。對(duì)基因組的細(xì)致研究發(fā)現(xiàn),，盡管新性染色體的起源時(shí)間及其晚近,，但僅在12萬年內(nèi)，該染色體對(duì)相互之間已經(jīng)積累了大量序列差異,。其中和Y染色體融合的新Y 染色體(neo-Y chromosome)由于不能重組,，已經(jīng)開始像所有的古老性染色體一樣開始逐漸退化：大約有80個(gè)基因左右已經(jīng)完全失去功能，有三分之一的基因表達(dá)開始紊亂（另見該物種的另一篇研究成果 http：//www.cell.com/current-biology/abstract/S0960-9822(12)00087-5）,，積累了額外的基因重排,。而它的B染色體的組裝序列，同時(shí)比對(duì)到了古老的性染色體和neo-sex染色體上,，提示B染色體的起源有可能是在兩正常染色體發(fā)生融合的過程中丟失的染色體片段產(chǎn)生的,。他們同時(shí)還在B染色體的序列上發(fā)現(xiàn)了一個(gè)有轉(zhuǎn)錄證據(jù)的基因片段，提示B染色體上有可能有功能序列,。
該工作最近在國際基因組學(xué)刊物BMC Genomics（IF 4.21）上發(fā)表以后,，很快受到同行關(guān)注，兩星期內(nèi)即成為高度訪問“highly access”和雜志首頁編輯首選“editor's pick”,。（生物谷Bioon.com）
doi:10.1186/1471-2164-13-109
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Deciphering neo-sex and B chromosome evolution by the draft genome of Drosophila albomicans
Qi Zhou, Hong-mei Zhu, Quan-fei Huang, Zhao-lin Xuan, Guo-jie Zhang, Li Zhao, Yun Ding, Scott Roy, Beatriz Vicoso, Jue Ruan, Yue Zhang, Ruo-ping Zhao, Bo Mu, Jiu-meng Min, Qing-hui Zhang, Jian-wen Li, Yin-ling Luo, Zu-heng Liang, Chen Ye, Rui-qiang Li, Xiu-qing Zhang, Jun Wang, Wen Wang and Doris Bachtrog
Background Drosophila albomicans is a unique model organism for studying both sex chromosome and B chromosome evolution. A pair of its autosomes comprising roughly 40% of the whole genome has fused to the ancient X and Y chromosomes only about 0.12 million years ago, thereby creating the youngest and most gene-rich 'neo-sex' system reported to date. This species also possesses recently derived B chromosomes that show non-Mendelian inheritance and significantly influence fertility.
Results We sequenced male flies with B chromosomes at 124.5-fold genome coverage, and inbred female flies derived from the same strain but without B's at 28.5-fold using next-generation sequencing. We assembled a female genome and placed 53% of the sequence and 85% of the annotated proteins into specific chromosomes, by comparison with the 12 Drosophila genomes. Despite its very recent origin, the non-recombining neo-Y chromosome shows various signs of degeneration, including a significant enrichment of non-functional genes compared to the neo-X, and an excess of tandem duplications relative to other chromosomes. We also characterized a B-chromosome linked scaffold that contains an actively transcribed unit and shows sequence similarity to the subcentromeric regions of both the ancient X and the neo-X chromosome.
Conclusions Our results provide novel insights into the very early stages of sex chromosome evolution and B chromosome origination, and suggest an unprecedented connection between the births of these two systems in D. albomicans.
