Department of Plant Biology, Uppsala BioCenter, Linnean Center of Plant Biology, Swedish University of Agricultural Sciences , Uppsala, Sweden.
Front Plant Sci. 2015 Mar 9;6:130. doi: 10.3389/fpls.2015.00130. eCollection 2015.
The endosperm is an ephemeral tissue surrounding the embryo that is essential for its development. Aside from the embryo nourishing function, the endosperm serves as a battlefield for epigenetic processes that have been hypothesized to reinforce transposable element silencing in the embryo. Specifically, global DNA demethylation in the central cell may serve to produce small RNAs that migrate to egg cell and embryo to induce de novo DNA methylation. The Polycomb Repressive Complex 2 (PRC2) is particularly targeted to DNA hypomethylated regions, possibly alleviating the negative effects associated with loss of DNA methylation in the endosperm. The functional requirement of the PRC2 in the endosperm can be bypassed by increasing the maternal genome dosage in the endosperm, suggesting a main functional role of the endosperm PRC2 in reducing sexual conflict. We therefore propose that the functional requirement of an endosperm PRC2 was coupled to the evolution of a sexual endosperm and mechanisms enforcing transposon silencing in the embryo. The evolutionary consequences of this scenario for genome expansion will be discussed.
胚乳是围绕胚胎的短暂组织，对胚胎的发育至关重要。除了胚胎营养功能外，胚乳还是表观遗传过程的战场，这些过程被假设可以加强转座元件在胚胎中的沉默。具体来说，中央细胞的全局 DNA 去甲基化可能有助于产生小 RNA，这些小 RNA 迁移到卵细胞和胚胎中诱导从头 DNA 甲基化。多梳抑制复合物 2 (PRC2) 特别靶向 DNA 低甲基化区域，可能减轻胚乳中 DNA 去甲基化相关的负面影响。胚乳中 PRC2 的功能要求可以通过增加胚乳中的母体基因组剂量来绕过，这表明胚乳 PRC2 的主要功能作用是减少性冲突。因此，我们提出胚乳 PRC2 的功能要求与有性胚乳的进化以及在胚胎中强制沉默转座子的机制相关联。我们将讨论这种情况下对基因组扩张的进化后果。
