Mobile Genetic Elements in Sexual and Ancient Asexual Taxa

转座遗传元件（te）在真核生物和原核生物中几乎普遍存在，在多细胞真核生物中已被发现构成了基因组的很大一部分：在人类中几乎占一半，在许多植物中甚至更多。它们的存在提出了与真核生物基因组结构、功能和进化相关的基本问题。本项目是基于这样一种观点，即通过比较性和无性分类群中te的特征和进化，可以了解更多关于te及其对基因组结构的影响。由于有性生殖在维持和限制真核生物种群中有害te方面的重要作用，蛭形轮虫假定的无性行为和其他非常不寻常的特征，如蛭形栖息地在脱水和补水周期中反复发生的DNA断裂和修复特征，使它们成为研究与te相关的基本问题的有希望的模型系统。这些问题包括它们在基因组中的增殖、区隔和分布，它们的拷贝数控制机制，它们的维持、动态和进化对双亲本繁殖的依赖，它们的驯化和在染色体末端维持中的可能作用，以及它们参与水平基因流，即TEs和其他DNA可以通过垂直下降以外的方式进入蛭形基因组。事实上，研究者对蛭形te的研究已经揭示了蛭形基因组中te和外源DNA的类型和基因组分布与性别分类群的显著差异，并为保护染色体及其末端免受降解的系统的运作和进化提出了新的想法。本研究的主要目标是：(i)研究类胶质细胞中富含TE的亚端粒簇，以确定端粒TE拷贝是否可能构成抵御入侵TE的基因组防御系统的一部分，并揭示端粒相关逆转录因子在这些簇的形成和功能中的作用；（ii）研究在蛭形类中发现的新型逆转录酶的起源和功能及其与基因组防御机制的可能关系；（iii）进一步检查蛭形和单体基因组中是否存在各种完整或残留的逆转录因子和富含te的端粒簇，并确定将候选的单体te引入蛭形基因组；（iv）继续努力实现稳定转化，以测试引入内部和端粒区域的TEs的增殖。这些特定的目标旨在描述蛭形生物基因组防御的基本机制及其对真核生物的影响。这项研究将对对基因组结构和进化问题感兴趣的生物学家以及公众感兴趣。调查人员将继续在媒体上讨论他们的研究，就像他们前几年在NPR的“联系”和“国家谈话：科学星期五”等节目中所做的那样。他们将为伍兹霍尔海洋生物实验室（MBL）的移动遗传元素集群活动做出贡献，并将参加召集年度东北移动DNA会议。该项目将继续依靠学生的参与，为本科生提供实践培训，并使他们有机会进入MBL独特的夏季环境，其中包括著名科学家的周五讲座，以及与来自世界各地的访问科学家和暑期课程学生互动的可能性。

Transposable genetic elements (TEs) are of virtually universal occurrence in eukaryotes and prokaryotes, and in multicellular eukaryotes have been found to constitute a substantial portion of the genome: nearly one-half in humans, and even more in many plants. Their existence raises fundamental questions related to eukaryotic genome structure, function, and evolution. This project is based on the view that much can be learned about TEs and their effects on genome structure by comparing their characteristics and evolution in sexual taxa with those in asexual taxa. Because of the important role of sexual reproduction in both the maintenance and limitation of deleterious TEs in eukaryotic populations, the putative asexuality and other highly unusual characteristics of bdelloid rotifers, such as the repeated DNA breakage and repair thought to occur during the cycles of desiccation and rehydration characteristic of bdelloid habitats, make them a promising model system for investigating fundamental problems associated with TEs. These problems include their proliferation, compartmentalization and distribution within genomes, the mechanisms of their copy number control, the dependence of their maintenance, dynamics and evolution on biparental reproduction, their domestication and possible role in the maintenance of chromosome ends, and their participation in horizontal gene flow, whereby TEs and other DNA may enter bdelloid genomes by means other than vertical descent. Indeed, the investigators' studies of bdelloid TEs have already revealed notable differences from sexual taxa in the types and genomic distribution of TEs and foreign DNA in bdelloid genomes, and have suggested novel ideas for the operation and evolution of systems that protect chromosomes and their ends from degradation. The principal objectives of the research are (i) to investigate subtelomeric TE-rich clusters in bdelloids in order to determine whether telomeric TE copies may constitute part of a genome defense system against invading TEs, and to uncover the role of telomere-associated retroelements in formation and function of these clusters; (ii) to investigate the origin and function of novel reverse transcriptases found in bdelloids and its possible relation to genome defense mechanisms; (iii) to further examine bdelloid and monogonont genomes for the presence of diverse intact or relict retroelements and telomeric TE-rich clusters, and to identify candidate monogonont TEs for introduction into bdelloid genomes; and (iv) to continue efforts to achieve stable transformation in order to test for proliferation of TEs introduced into internal and telomeric regions. These specific objectives are aimed at characterizing fundamental mechanisms of genome defense in bdelloids and their implications for eukaryotes generally. The research will be of interest to biologists interested in problems of genome structure and evolution, as well as to the general public. The investigators will continue to discuss their research in the media, as they have done in previous years on programs such as NPR's "The Connection" and "Talk of the Nation: Science Friday." They will contribute to the activities of Mobile Genetic Element Cluster at the Marine Biological Laboratory (MBL) at Woods Hole, and will participate in convening the annual Northeast Mobile DNA meeting. The project will continue to rely on student involvement, providing undergraduates with hands-on training and with access to the unique summer environment at the MBL, which includes Friday lectures by eminent scientists and the possibility to interact with visiting scientists and summer course students from around the world.