The jojoba genome reveals wide divergence of the sex chromosomes in a dioecious plant

背景回顾：Most flowering plants are hermaphrodites but around 6% of species are dioecious having separate male and female plants. 

提出问题：Sex chromosomes and some sex specific genes have been reported in plants, but the genome sequences have not been compared.

主要研究：We now report the genome sequence of male and female jojoba (Simmondsia chinensis) plants revealing a very large difference in the sex chromosomes. 

结果1-基因组大小差异：The male genome assembly was 832 Mb and the female 822 Mb. This was explained by the large size differences in the Y chromosome (37.6 Mb) compared with the X chromosome (26.9 Mb). 

结果2-Y染色体的2个大片段插入：Relative to the X chromosome, the Y chromosome had 2 large insertions each of more than 5 Mb containing more than 400 genes. Many of the genes in the chromosome specific regions were novel. These male specific regions included many flowering-related and stress response genes. Smaller insertions found only in the X chromosome totalled 877 kb. 

结果3-性染色体适应与演化：The wide divergence of the sex chromosomes suggests a long period of adaptation to diverging sex specific roles. Male and female plants may have evolved to accommodate factors such as differing reproductive resource allocation requirements under the stress of the desert environment in which the plants are found. The sex determining regions accumulate genes beneficial to each sex. This has required the evolution of many more novel sex specific genes than has been reported for other organisms. 

结论：This suggest that dioecious plants provide a novel source of genes for manipulation of reproductive performance and environmental adaptation in crops.

 摘 要 

大多数的有花植物是雌雄同株的，但也有大约6%的有花植物属于雌雄异株，即有雄株和雌株的分别。植物中已报道了性染色体和一些性别特异性基因，但是有关基因组层面的比较研究还比较少见。本文中，作者报道了霍霍巴树的雄株和雌株基因组，发现了其性染色体间存在较大的差异。雄株的基因组大小约为832Mb，雌株约为822Mb，这是由于雄株的Y染色体大小约为37.6Mb，而雌株的X染色体仅为26.9Mb。与X染色体相比，Y染色体有两个长度超多5Mb的大片段插入，每一段插入包含超过了400个基因。许多在染色体特异区域的基因都是新基因。这些雄株特异性区域包含很多与开花和胁迫响应相关的基因。而在X染色体中发现的插入仅为877kb。霍霍巴树性染色体之间巨大的差异说明了其已经适应了不同性别角色很长一段时间。雄株和雌株已经演化出了很多适应性的因素，比如在沙漠环境胁迫下的差异性生殖资源配置。性别决定区域积累了有益于各自性别的基因。而这就需要演化出比其他生物更多的新的性别特异性基因。这表明雌雄异株植物为调控作物的生殖性能和环境适应提供了新的基因来源。