Mobile Genetic Elements in Sexual and Ancient Asexual Taxa

这是一项结构，系统发育，活动模式的调查，并在古老的无性动物门蛭形纲的转座逆转录因子失活。蛭形轮虫是一种小型无脊椎动物，分布在世界各地，大约有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，即它们是唯一一种没有减数分裂或有性生殖的长期进化有广泛形态学、细胞遗传学和分子证据的动物类群。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的因素提供了一个实验系统。蛭形轮虫和所有有性生殖的真核生物之间的一个显著和根本的区别是蛭形轮虫缺乏主要的反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，这些元素最终将导致宿主谱系灭绝，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反转录转座子对新产生的无性系的消除。因此，研究者试图测试有害的逆转录转座子在蛭形进化的早期丢失或失活，如果存在任何活性的逆转录因子，它们并不是显著有害的。具体来说，该研究将(i)表征在蛭形基因组中存在的非常不寻常的penelope样逆转录因子的结构、系统发育和活性状态；（ii）发展适用于蛭形轮虫和单体轮虫的性腺显微注射和微粒轰击方法；（iii）采用上述方法将活性的LINE-like逆转录因子引入到蛭形和单体基因组中，以测试蛭形特异性灭活逆转录转座子的机制是否存在。这个项目应该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植物和动物中。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.