Youngson Neil A, Kocialkowski Sylvia, Peel Nina, Ferguson-Smith Anne C
Department of Anatomy, University of Cambridge, Downing St, Cambridge, CB23DY, UK.
J Mol Evol. 2005 Oct;61(4):481-90. doi: 10.1007/s00239-004-0332-0. Epub 2005 Sep 12.
Ty3/gypsy retrotransposons are rare in mammalian genomes despite their abundance in invertebrate and other vertebrate classes. Here we identify a family of nine conserved mammalian genes with homology to Ty3/gypsy retrotransposons but which have lost their ability to autonomously retrotranspose. Of these, five map to the X chromosome while the remaining four are autosomal. Comparative phylogenetic analyses show them to have strongest homology to the sushi-ichi element from Fugu rubripes. Two of the autosomal gene members, Peg10 and Rtl1, are known to be imprinted, being expressed from the paternally inherited chromosome homologue. This suggests, consistent with the host-parasite response theory of the evolution of the imprinting mechanism, that parental-origin specific epigenetic control may be mediated by genomic "parasitic" elements such as these. Alternatively, these elements may preferentially integrate into regions that are differentially modified on the two homologous chromosomes such as imprinted domains and the X chromosome and acquire monoallelic expression. We assess the imprinting status of the remaining autosomal members of this family and show them to be biallelically expressed in embryo and placenta. Furthermore, the methylation status of Rtl1 was assayed throughout development and was found to resemble that of actively, silenced repetitive elements rather than imprinted sequences. This indicates that the ability to undergo genomic imprinting is not an inherent property of all members of this family of retroelements. Nonetheless, the conservation but functional divergence between the different members suggests that they have undergone positive selection and acquired distinct endogenous functions within their mammalian hosts.
Ty3/gypsy逆转座子在哺乳动物基因组中很少见，尽管它们在无脊椎动物和其他脊椎动物类别中大量存在。在这里，我们鉴定出一个由九个保守的哺乳动物基因组成的家族，它们与Ty3/gypsy逆转座子具有同源性，但已失去自主逆转座的能力。其中，五个基因定位于X染色体，其余四个为常染色体基因。比较系统发育分析表明，它们与红鳍东方鲀的sushi-ichi元件具有最强的同源性。已知两个常染色体基因成员Peg10和Rtl1是印记基因，从父系遗传的染色体同源物中表达。这表明，与印记机制进化的宿主-寄生虫反应理论一致，亲本来源特异性表观遗传控制可能由这些基因组“寄生”元件介导。或者，这些元件可能优先整合到两条同源染色体上差异修饰的区域，如印记域和X染色体，并获得单等位基因表达。我们评估了该家族其余常染色体成员的印记状态，发现它们在胚胎和胎盘中双等位基因表达。此外，在整个发育过程中检测了Rtl1的甲基化状态，发现其类似于活跃沉默的重复元件而非印记序列。这表明经历基因组印记的能力并非该逆转座子家族所有成员的固有特性。尽管如此，不同成员之间的保守性但功能差异表明它们经历了正选择，并在其哺乳动物宿主中获得了独特的内源性功能。
