性染色体和B染色体（相对于正常染色体而言不遵循孟德尔遗传分裂规律的染色体）的演化一直是经典遗传学长期未曾研究透彻的重要问题。有意思的是，在一种叫做银额果蝇（Drosophila albomicans）的果蝇物种（如图），新近演化出了非常年轻的性染色体和B染色体，使其成为研究两者的绝佳材料。该果蝇仅有3对染色体，其古老的性染色体和一对常染色体发生融合，使得该对常染色体和性别连锁，相互之间无法重组，成为拥有和古老性染色体同样遗传模式的‘新性染色体’(neo-sex chromosome)。另外，90年代初期王文研究员等人对该果蝇的B染色体研究发现，特定数目的B染色体将对银额果蝇的后代产生显著影响，直接质疑了一直以来B染色体完全只是‘自私遗传因子’，没有任何功能的传统观点。而这两种系统的起源时间仅仅是12万年。
最近，昆明动物所已毕业博士生周琦和赵莉，在王文研究员指导下，通过和深圳华大基因研究院和美国加州伯克利大学的合作，对该果蝇进行了全基因组的测定。他们运用新一代Illumina测序技术，快速完成了该果蝇基因组的从头组装，使它成为世界上第一个完全应用新技术测序组装的果蝇基因组。对基因组的细致研究发现，尽管新性染色体的起源时间及其晚近，但仅在12万年内，该染色体对相互之间已经积累了大量序列差异。其中和Y染色体融合的新Y 染色体(neo-Y chromosome)由于不能重组，已经开始像所有的古老性染色体一样开始逐渐退化：大约有80个基因左右已经完全失去功能，有三分之一的基因表达开始紊乱（另见该物种的另一篇研究成果 http：//www.cell.com/current-biology/abstract/S0960-9822(12)00087-5），积累了额外的基因重排。而它的B染色体的组装序列，同时比对到了古老的性染色体和neo-sex染色体上，提示B染色体的起源有可能是在两正常染色体发生融合的过程中丢失的染色体片段产生的。他们同时还在B染色体的序列上发现了一个有转录证据的基因片段，提示B染色体上有可能有功能序列。
该工作最近在国际基因组学刊物BMC Genomics（IF 4.21）上发表以后，很快受到同行关注，两星期内即成为高度访问“highly access”和杂志首页编辑首选“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.
