Mobile Genetic Elements in Sexual and Ancient Asexual Taxa

本文对轮虫门蛭总科古老无性类群中转座逆转录因子的结构、发生、活动方式和失活进行了研究。 蛭形轮虫是一种小型无脊椎动物，分布于世界各地，约有360种，主要分布在淡水和潮湿的陆地栖息地。它们的重要性源于这样一个事实，即它们是唯一一个有大量形态学、细胞遗传学和分子证据证明没有减数分裂或有性生殖的长期进化的动物群体。因此，蛭形轮虫为研究几乎所有无性种群的早期灭绝以及减数分裂和性别的普遍存在提供了一个实验系统。蛭形轮虫与所有有性繁殖的真核生物的一个显著和根本的区别是，蛭形轮虫缺乏主要逆转录转座子超家族的转座遗传元件。这种缺乏与基于理论的预测是一致的，即这些元素可以在有性繁殖的物种中稳定地存在，但在无性繁殖的物种中，这些元素最终会使宿主谱系灭绝，或者失活或丢失。因此，真核生物中几乎普遍存在有性生殖的一个主要因素可能是有害的逆转录转座子引起的新出现的无性系的消除。因此，研究者试图检验有害的反转录转座子在蛭形虫进化早期丢失或失活的暗示，以及如果存在任何活性反转录元件，它们不是显著有害的。具体来说，研究将（i）表征蛭形虫基因组中存在的高度不寻常的Penelope样反转录元件的结构、发生和活性状态; ㈡开发适用于蛭形轮虫和单殖体轮虫的性腺显微注射和微粒轰击方法;及（iii）采用该等方法引入活性LINE-为了测试蛭类动物特有的逆转录转座子的机制的存在，这个项目将有助于揭示为什么有性生殖和减数分裂存在于几乎所有的动植物中。 它也将加深我们对这一基本生物现象的进化的理解。

This is an investigation of the structure, phylogeny, mode of activity, and inactivation of transposable retroelements in the ancient asexual Class Bdelloidea of the Phylum Rotifera. Bdelloid rotifers are small invertebrate animals of worldwide distribution represented by approximately 360 described species and found mainly in freshwater and moist terrestrial habitats. Their importance stems from the fact that they are the only animal group for which there is extensive morphological, cytogenetic, and molecular evidence for long-term evolution without meiosis or sexual reproduction. Bdelloid rotifers therefore provide an experimental system for investigation of the factors responsible for the early extinction suffered by nearly all asexual populations and for the ubiquity of meiosis and sex.A striking and fundamental difference between bdelloid rotifers and all sexually-reproducing eukaryotes that have been investigated is that bdelloids lack transposable genetic elements of the major retrotransposon superfamilies. Such lack is consistent with the theoretically-based prediction that these elements can be stably harbored in sexually-reproducing species but that in asexuals such elements will eventually either drive the host lineage to extinction or else be inactivated or lost. Thus, a major factor responsible for the nearly universal prevalence of sexual reproduction in eukaryotes may be the elimination, caused by deleterious retrotransposons, of newly arising asexual lines. The investigator therefore seeks to test the implication that deleterious retrotransposons were lost or inactivated early in bdelloid evolution and that if any active retroelements are present, they are not significantly deleterious.Specifically, the research will (i) characterize the structure, phylogeny, and activity status of the highly unusual Penelope-like retroelement present in bdelloid genomes; (ii) develop gonadal microinjection and microparticle bombardment methods applicable to bdelloid and monogonont rotifers; and (iii) employ such methods to introduce active LINE-like retroelements into bdelloid and monogonont genomes in order to test for the existence of mechanisms specific to bdelloids that inactivate retrotransposons.This project should help reveal why sexual reproduction and meiosis are found in virtually all plants and animals. It will also deepen our understanding of the evolution of this basic biological phenomenon.