Mobile Genetic Elements in Sexual and Ancient Asexual Taxa

本文对轮虫纲中古老的无性纲的转座型反转录元件的结构、系统发育、活性模式和失活进行了研究。蜈蚣类轮虫是一种分布在世界各地的小型无脊椎动物，描述了大约360个物种，主要分布在淡水和潮湿的陆地生境中。它们的重要性源于这样一个事实，即它们是唯一有广泛的形态、细胞遗传学和分子证据表明没有减数分裂或有性繁殖的长期进化的动物群体。因此，蜈蚣类轮虫提供了一个实验系统，用于研究导致几乎所有无性种群早期灭绝的因素，以及减数分裂和性别的普遍存在。蜈蚣类轮虫与所有已被研究的有性繁殖真核生物之间的一个显著和根本的区别是，蜈蚣类缺乏主要反转录转座子超家族的可转座遗传元件。这种缺乏与基于理论的预测一致，即这些元素可以稳定地藏匿在有性繁殖物种中，但在无性繁殖物种中，这些元素最终要么会导致宿主谱系灭绝，要么会失活或消失。因此，导致真核生物中几乎普遍存在的有性繁殖的一个主要因素可能是由有害的反转录转座子引起的新出现的无性品系的消除。因此，研究人员试图测试有害的反转录转座子在蜈蚣进化早期丢失或失活的含义，以及如果存在任何活跃的逆转录元件，它们并不是显著有害的。具体地说，这项研究将(I)表征存在于蜈蚣基因组中的极不寻常的Penelope样反转录元件的结构、系统发生和活性状态；(Ii)开发适用于蜈蚣和单基因轮虫的性腺显微注射和微粒子轰击方法；以及(Iii)使用这种方法将活跃的线状逆转录元件引入蜈蚣和单体基因组中，以测试是否存在使反转录转座子失活的蜈蚣特有的机制。这个项目应该有助于揭示为什么有性繁殖和减数分裂在几乎所有的动植物中都能找到。它还将加深我们对这一基本生物现象进化的理解。

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.