Department of Zoology, The University of Melbourne, Victoria 3010, Australia.
Philos Trans R Soc Lond B Biol Sci. 2013 Jan 5;368(1609):20120151. doi: 10.1098/rstb.2012.0151.
Genomic imprinting is widespread in eutherian mammals. Marsupial mammals also have genomic imprinting, but in fewer loci. It has long been thought that genomic imprinting is somehow related to placentation and/or viviparity in mammals, although neither is restricted to mammals. Most imprinted genes are expressed in the placenta. There is no evidence for genomic imprinting in the egg-laying monotreme mammals, despite their short-lived placenta that transfers nutrients from mother to embryo. Post natal genomic imprinting also occurs, especially in the brain. However, little attention has been paid to the primary source of nutrition in the neonate in all mammals, the mammary gland. Differentially methylated regions (DMRs) play an important role as imprinting control centres in each imprinted region which usually comprises both paternally and maternally expressed genes (PEGs and MEGs). The DMR is established in the male or female germline (the gDMR). Comprehensive comparative genome studies demonstrated that two imprinted regions, PEG10 and IGF2-H19, are conserved in both marsupials and eutherians and that PEG10 and H19 DMRs emerged in the therian ancestor at least 160 Ma, indicating the ancestral origin of genomic imprinting during therian mammal evolution. Importantly, these regions are known to be deeply involved in placental and embryonic growth. It appears that most maternal gDMRs are always associated with imprinting in eutherian mammals, but emerged at differing times during mammalian evolution. Thus, genomic imprinting could evolve from a defence mechanism against transposable elements that depended on DNA methylation established in germ cells.
基因组印迹在真兽类哺乳动物中广泛存在。有袋类哺乳动物也具有基因组印迹，但印迹的基因数量较少。长期以来，人们认为基因组印迹与哺乳动物的胎盘形成和/或胎生有关，尽管这两者都不限于哺乳动物。大多数印迹基因在胎盘内表达。尽管产卵的单孔目哺乳动物的胎盘寿命很短，只能从母体向胚胎传递营养，但在这些动物中没有发现基因组印迹的证据。在新生儿的主要营养来源——乳腺中，也存在后天的基因组印迹。然而，在所有哺乳动物中，人们很少关注新生儿的营养来源，即乳腺。差异甲基化区域（DMR）作为每个印迹区域的印迹控制中心，在印迹区域中起着重要作用，这些区域通常包含父系和母系表达的基因（PEG 和 MEG）。DMR 是在雄性或雌性生殖细胞中建立的（gDMR）。全面的比较基因组研究表明，两个印迹区域 PEG10 和 IGF2-H19 在有袋类和真兽类中都得到了保守，PEG10 和 H19 的 DMR 至少在 1.6 亿年前出现在有胎盘哺乳动物的祖先中，这表明基因组印迹是在有胎盘哺乳动物的进化过程中产生的。重要的是，这些区域已知与胎盘和胚胎生长密切相关。似乎大多数母系 gDMR 总是与真兽类哺乳动物的印迹有关，但在哺乳动物进化过程中出现的时间不同。因此，基因组印迹可能是从一种依赖于生殖细胞中建立的 DNA 甲基化的防御机制进化而来的，这种机制是为了抵御转座元件。
