本期封面所示为在共焦显微镜下看到的结合在一起的一对草履虫细胞：O-交配型细胞经改造表达荧光标记的纤毛蛋白(在图上用黄色表示)，O-型和E-型细胞都以“TAP952 anti-monoglycylated”微管蛋白抗体(在图上用红色表示)来装饰。原生动物草履虫(作为一种典型的纤毛虫受到广泛研究)在大部分情况下通过“二分裂”进行无性繁殖，但在某些条件下将会进行有性生殖。1937年发现了两个交配型E和O，但交配型的母方遗传的分子基础只是现在才得到阐释。Eric Meyer 及同事通过“第四双小核草履虫”(Paramecium tetraurelia)发现，交配型E取决于跨膜蛋白mtA的表达，默认交配型O是在发育过程中由mtA启动子被scnRNAs的切除决定的。scnRNAs是一类小‘scan’ RNA，它们在有性生殖过程中通过识别和切除可转座的元素来重新编程草履虫的基因组。涉及另一不同基因mtB的一个相似的切换机制在该草履虫的一个姐妹物种P. septaurelia中是独立演化形成的，说明scnRNA通道的功能改变(exaptation)也许是草履虫不同状态的跨带表观遗传的一个普遍机制。
Abstract
In the ciliate Paramecium, transposable elements and their single-copy remnants are deleted during the development of somatic macronuclei from germline micronuclei, at each sexual generation. Deletions are targeted by scnRNAs, small RNAs produced from the germ line during meiosis that first scan the maternal macronuclear genome to identify missing sequences, and then allow the zygotic macronucleus to reproduce the same deletions. Here we show that this process accounts for the maternal inheritance of mating types in Paramecium tetraurelia, a long-standing problem in epigenetics. Mating type E depends on expression of the transmembrane protein mtA, and the default type O is determined during development by scnRNA-dependent excision of the mtA promoter. In the sibling species Paramecium septaurelia, mating type O is determined by coding-sequence deletions in a different gene, mtB, which is specifically required for mtA expression. These independently evolved mechanisms suggest frequent exaptation of the scnRNA pathway to regulate cellular genes and mediate transgenerational epigenetic inheritance of essential phenotypic polymorphisms.