RESEARCH-PGR: Genomic mechanisms of domesticating a Y chromosome in papaya

许多植物在同一株植物上有雄花和雌花。另一方面，木瓜植物是性别不同的：木瓜植物可能是雄性或雌性或所谓的雌雄同体，即在同一花中既有雄性又有雌性的植物。野生木瓜只有雄性和雌性，而栽培木瓜大多是雌雄同体，这是大约4000年前在中美洲从性别反转的雄性木瓜中挑选出来的。了解这种罕见的性别生物学是如何被控制的，以及是否可以利用雌雄同体来控制开花，将极大地有利于番木瓜农业。目前已知，这些树之间的差异是由于不同的Y染色体：雌性有两条X染色体，雄性有X和Y染色体，雌雄同体有X和修饰的Yh。也有证据表明，在中美洲番木瓜驯化的早期，雌雄同体染色体可能已经进化。该项目将确定在具有不同染色体类型的植物中发生了哪些因素和全基因组变化。一旦确定，植物就可以被改造成没有X染色体的雌雄同体，因此没有类型分离。这种材料将加速识别控制雄花形成的因素，雄花是从雄性回到雌雄同体的目标。本科生将接受培训，在研究这种重要作物和开发育种工具的同时获得实践研究经验。该项目将通过视频向种植者展示有关木瓜生物学和控制开花方法的信息。木瓜性染色体是第一个被测序的三重奏（Y， Yh和X）。独特的三组性染色体和丰富的基因组和生物资源使我们能够研究从雄性到雌雄同体的逆转，这是性染色体进化早期罕见的事件。我们的目标是了解控制Yh染色体驯化的基因组和进化机制。第一个目的是建立木瓜YY致死基因的分子基础，恢复YhYh雌雄同体和YhY雄性基因型的育性，对yh特异性基因进行功能分析。一个强有力的候选基因已经被确定，并正在通过转化为雌雄同体植物来测试其功能。一旦产生有活力的YhYh雌雄同体，将通过精细作图鉴定Y染色体上影响花形态和花粉育性的40个突变。第二个目的是确定抑制雄花心皮发育的性别决定基因和导致雌雄同体性别逆转的突变事件。该项目的重点是为鉴定抑制雄花心皮发育的性别决定基因而产生的12个性别逆转突变体，一旦YYh生育力恢复，将进行基于图谱的基因克隆。通过对YY和Yh基因型基因表达的对比分析，揭示性别决定基因控制的差异表达基因和基因网络。所产生的靶基因和基因组资源将导致木瓜新品种的选择，而不是性别类型的分离和生产力的提高

Many plants have male and female floral parts housed on the same plant. Papaya plants, on the other hand, are gender-distinct: papaya plants may be male or female or so-called hermadrodites, which are plants that both male and female parts in the same flowers. Wild papayas are male and female only, and cultivated papayas are mostly hermaphrodite, which was selected from sex reversed male papaya about 4,000 years ago in Central America. Understanding how this rare gender biology is controlled, and whether hermaphroditism could be harnessed for control of flowering, would greatly benefit agriculture of papaya. It is currently known that the difference among these trees is due to different Y chromosomes: females have two X chromosomes, males have an X and Y and hermaphrodites an X and modified Yh. There is also evidence that hermaphrodite chromosome may have evolved during the earliest days of domestication of papaya in Central America. The project will identify what factors and genome-wide changes are occurring in the plants with varying chromosome types. Once identified, plants could be engineered to produce hermaphrodites with no X chromosome, hence no segregation of types. This material will expedite the identification of the factors that control the formation of male flowers, the target for sex reversal from male back to hermaphrodite. Undergraduate students will be trained to gain hands-on research experience while working with this important crop and developing breeding tools. The project will display information for growers on informative videos about papaya biology and ways to control flowering. Papaya sex chromosomes are the first trio (Y, Yh, and X) to be sequenced. The unique set of three sex chromosomes and abundant genomic and biological resources allow us to investigate the reversion from male to hermaphrodite, a rare event during the early stage of sex chromosome evolution. Our goal is to understand the genomic and evolutionary mechanisms governing the domestication of the Yh chromosome. The first aim is to establish the molecular basis for papaya YY lethality gene and restore fertility of YhYh hermaphrodite and YhY male genotypes for functional analysis of Yh-specific genes. A strong candidate gene has been identified and is being tested for function by being transformed into hermaphrodite plants. Once viable YhYh hermaphrodites are generated, the 40 mutations on Y chromosome affecting flower morphology and pollen fertility will be identified through fine mapping. The second aim is to identify the sex determination gene suppressing carpel development in male flowers and the mutation event leading to the sex reversal from male back to hermaphrodite. The project is focusing on 12 sex reversal mutants generated for identification of the sex determination gene suppressing carpel development in male flowers, and will conduct map based gene cloning once the YYh fertility is restored. Comparative analysis of gene expression between YY and Yh Yh genotypes will reveal differentially expressed genes and gene networks controlled by the sex determination gene. The target genes and genomic resources generated will lead to selection of new papaya varieties without segregation of sex types and with improved productivity