Mobile Genetic Elements in Sexual and Ancient Asexual Taxa

摘要这项研究的目的是确定该发现的普遍性和意义，这一发现与轮虫纲的其他轮虫和所有其他已被研究的真核生物形成鲜明的对比，似乎不包含反转录转座子。证实没有反转录转座子，也可能存在DNA转座子，将代表着与这些元件在所有其他真核生物中普遍存在的不同寻常的背离，因此有可能为可移动遗传元件的进化、维持和作用提供新的见解。此外，这一发现将具有根本的意义，因为蜈蚣类轮虫的另一个非常不寻常且可能相关的属性，即它们明显的无性。在拟议的研究中，将遵循两种互补的实验方法。第一种方法利用聚合酶链式反应(PCR)和嵌套的简并引物来扩增特定的保守结构域，这些保守结构域对每一类主要的RNA和DNA转座子都有诊断作用。得到的预期大小的扩增产物将被克隆并测序，以测试它们的特性。这种筛选将包括不同种类的蜈蚣类轮虫，并作为对照，包括不同的单一轮虫和以前没有检查过此类元素的某些其他动物门。在接下来的第二种方法中，将对粘粒文库中大约1兆碱基的蜈蚣DNA进行测序，以确定无法设计有效的PCR引物的可移动元件，包括可能偏离了PCR和其他溶液杂交方法的检测能力的古老的非活性元件。有性繁殖的丧失是进化的死胡同，与之相反的是，整个蜈蚣纲似乎已经进化了数千万年，没有雄性、减数分裂或任何形式的有性繁殖。因此，如果发现瓢虫缺乏RNA和/或DNA转座子，则表明这些元件在进化过程中不会保留在无性品系中，和/或它们的丢失或失活是瓢虫成功避免灭绝和经历广泛进化而不进行有性繁殖的先决条件。因此，对这一新系统中移动元素的状态的研究也可能揭示这些元素的存在与有性繁殖的进化和维持之间的基本关系。

Abstract The goal of this research is to determine the generality and significance of the finding that rotifers of the Class Bdelloidea, in striking contrast to other classes of the Phylum Rotifera and all other eukaryotes that have been examined, appear not to contain retrotransposons. Confirmation of the absence of retrotransposons and, possibly, of DNA transposons as well, would represent an extraordinary departure from the universal occurrence of such elements in all other eukaryotes and would therefore have the potential of providing new insights into the evolution, maintenance, and role of mobile genetic elements. Moreover, such a finding would be of fundamental interest because of another highly unusual and possibly related attribute of bdelloid rotifers, their apparent asexuality. Two complementary experimental approaches will be followed in the proposed research. The first utilizes polymerase chain reaction (PCR) with nested sets of degenerate primers designed to amplify specific conserved domains that are diagnostic for each of the major classes of RNA and DNA transposons. Resulting amplicons of the expected size will be cloned and sequenced to test their identity. Such screening will include diverse species of bdelloid rotifers and, as controls, diverse monogonont rotifers and certain other animal phyla not previously examined for such elements. In the second approach to be followed, approximately one megabase of bdelloid DNA from cosmid libraries will be sequenced in order to identify mobile elements for which no effective PCR primers can be designed, including ancient inactive elements that may have diverged beyond the detection capability of PCR and other solution hybridization methods. Contrary to the generalization that the loss of sexual reproduction is a dead end in evolution, the entire Class Bdelloidea appears to have evolved for tens of millions of years without males, meiosis, or any form of sexual reproduction. A finding that bdelloids lack RNA and/or DNA transposons would therefore suggest that such elements are not retained in asexual lines over evolutionary time periods and/or that their loss or inactivation was a pre-condition for the success of bdelloids in avoiding extinction and undergoing extensive evolution without sexual reproduction. Thus, investigation of the status of mobile elements in this novel system may also reveal fundamental relationships between the presence of such elements and the evolution and maintenance of sexual reproduction.