Rice Research Institute of Sichuan Agricultural University, Chengdu 611130, China.
J Genet Genomics. 2013 May 20;40(5):239-47. doi: 10.1016/j.jgg.2013.04.003. Epub 2013 Apr 20.
Genomic imprinting (or imprinting) refers to an epigenetic phenomenon by which the allelic expression of a gene depends on the parent of origin. It has evolved independently in placental mammals and flowering plants. In plants, imprinting is mainly found in endosperm. Recent genome-wide surveys in Arabidopsis, rice, and maize identified hundreds of imprinted genes in endosperm. Since these genes are of diverse functions, endosperm development is regulated at different regulatory levels. The imprinted expression of only a few genes is conserved between Arabidopsis and monocots, suggesting that imprinting evolved quickly during speciation. In Arabidopsis, DEMETER (DME) mediates hypomethylation in the maternal genome at numerous loci (mainly transposons and repeats) in the central cell and results in many differentially methylated regions between parental genomes in the endosperm, and subsequent imprinted expression of some genes. In addition, histone modification mediated by Polycomb group (PcG) proteins is also involved in regulating imprinting. DME-induced hypomethylated alleles in the central cell are considered to produce small interfering RNAs (siRNAs) which are imported to the egg to reinforce DNA methylation. In parallel, the activity of DME in the vegetative cell of the male gametophyte demethylates many regions which overlap with the demethylated regions in the central cell. siRNAs from the demethylated regions are hypothesized to be also transferred into sperm to reinforce DNA methylation. Imprinting is partly the result of genome-wide epigenetic reprogramming in the central cell and vegetative cell and evolved under different selective pressures.
基因组印记（或印记）是指一种表观遗传现象，其中基因的等位基因表达取决于亲本的来源。它在胎盘哺乳动物和开花植物中独立进化。在植物中，印记主要存在于胚乳中。最近在拟南芥、水稻和玉米中的全基因组调查鉴定了胚乳中数百个印记基因。由于这些基因具有多种功能，胚乳发育在不同的调控水平上受到调节。在拟南芥和单子叶植物之间，只有少数基因的印记表达是保守的，这表明印记在物种形成过程中迅速进化。在拟南芥中，DEMETER（DME）在中央细胞中的许多基因座（主要是转座子和重复序列）中介导母体基因组的低甲基化，导致胚乳中亲本基因组之间存在许多差异甲基化区域，随后一些基因的印记表达。此外，多梳组（PcG）蛋白介导的组蛋白修饰也参与调节印记。中央细胞中 DME 诱导的低甲基化等位基因被认为产生小干扰 RNA（siRNA），这些 siRNA 被导入卵中以增强 DNA 甲基化。同时，雄性配子体营养细胞中的 DME 活性使许多与中央细胞中去甲基化区域重叠的区域去甲基化。来自去甲基化区域的 siRNA 也被假设转移到精子中以增强 DNA 甲基化。印记是中央细胞和营养细胞中全基因组表观遗传重编程的部分结果，并且在不同的选择压力下进化。
