MOLECULAR GENETICS OF NUCLEOLAR DOMINANCE

核仁优势是一种表观遗传现象，在植物界和动物界都存在，描述了在物种间杂交或异源多倍体中，从一个祖先遗传的核糖体RNA基因的转录，而不是另一个祖先的转录。长期以来，人们一直认为显性基因是选择性激活的。然而，最近的证据表明，显性不足的基因被选择性地沉默。胞嘧啶甲基化和组蛋白去乙酰化是抑制途径中的伙伴，但尚不清楚它们是并行作用还是串联作用。目前也不清楚染色质修饰是否作用于单个rRNA基因、rRNA基因聚集的多兆碱基染色体区域(NORs)或其他调节点。这些问题将用拟南芥作为我们的模型系统来解决，拟南芥是一种异源四倍体，它结合了拟南芥和拟南芥的基因组，来自拟南芥的rRNA基因在转录上是活跃的，而拟南芥的rRNA基因是沉默的。为了验证需要在前体基因组中的胞嘧啶甲基化模式来建立核仁显性的假设，拟南芥胞嘧啶低甲基化突变体ddm1将作为亲本用于重建拟南芥品系。将检验组蛋白去乙酰化导致胞嘧啶去甲基化或胞嘧啶去甲基化导致组蛋白超乙酰化的可能性，以检验这些染色质修饰在核仁显性控制中串联起作用的假设。RRNA基因独立于染色体位置受到调控的假设将通过确定在新形成的甘蔗品系中异位rRNA转基因是否被沉默来检验。在一个相关的目标中，我们将确定沉默是否仅限于NORs，或者它是否传播到与NORs相邻的邻近基因。这些实验是我们长期努力的一部分，目的是了解控制rRNA基因转录的积极和消极调控机制。了解核仁显性也可能有助于揭示其他表观遗传现象，如X染色体、前病毒和转座元件失活或配子印记。这种理解可能会带来实际的好处。例如，发育沉默基因的去抑制可能有可能挽救由于成人特有的同源基因缺陷而导致的疾病表型。我们在核仁显性方面的工作有望为我们理解染色体对基因表达的控制提供新的信息。

Nucleolar dominance is an epigenetic phenomenon that occurs in both the plant and animal kingdoms and describes the transcription of ribosomal RNA genes inherited from one progenitor, but not the other, in an interspecies hybrid or allopolyploid. It has long been thought that dominant genes are selectively activated. However, recent evidence suggests that under-dominant genes are selectively silenced. Cytosine methylation and histone deacetylation are partners in the repression pathway but it is not known if they act in parallel or in series. It is also not clear if chromatin modifications act on the individual rRNA genes, the multi-megabase chromosomal regions where rRNA genes are clustered (NORs), or other regulatory loci. These questions will be addressed using, as our model system, Arabidopsis suecica, an allotetraploid that combines the genomes of A. thaliana and C. arenosa (also known as A. arenosa) In A. suecica, rRNA genes derived from C. arenosa are transcriptionally active and A. thaliana rRNA genes are silenced. To test the hypothesis that cytosine methylation patterns in the progenitor genomes is needed to establish nucleolar dominance, the A. thaliana cytosine hypomethylation mutant ddm1 will be used as one parent to recreate A. suecica lines. The possibility that histone deacetylation causes cytosine demethylation or that cytosine demethylation causes histone hyperacetylation will be examined to test the hypothesis that these chromatin modifications act in series in the control of nucleolar dominance. The hypothesis that rRNA genes are regulated independent of chromosomal location will be tested by determining if rRNA transgenes at ectopic locations are silenced in newly formed A. suecica lines. In a related aim, we will determine if silencing is restricted to the NORs or if it spreads to neighboring genes adjacent to the NORs. These experiments are part of our long-term effort to understand the positive and negative regulatory mechanisms that control rRNA gene transcription. Understanding nucleolar dominance may also shed light on other epigenetic phenomena such as X-chromosome, provirus, and transposable element inactivation or gametic imprinting. Practical benefits may come from such understanding. For instance, derepression of developmentally silenced genes could have potential for rescuing disease phenotypes due to defects in adult-specific paralogs. Our work on nucleolar dominance is expected to contribute new information to our understanding of chromosomal controls of gene expression.