Collaborative Research: Molecular Genetic Studies of Bdelloid Rotifers

尽管有许多假设，但对于是什么维持有性生殖以及是什么导致放弃有性生殖的谱系的早期灭绝，还没有达成普遍的共识。解决这一基本问题的一个有希望的方法是研究蛭形纲的轮虫，这是一种非常不寻常的无脊椎动物，包括大约370种已描述的物种，其中雄性、雌雄同体、退化的雄性结构和减数分裂都是未知的，然而，它却持续存在，成功地进化和多样化了数千万年。该项目将研究蛭形虫无性与基因组结构之间的关系，以及蛭形虫抵御自然栖息地干燥特征的能力和最近发现的从极高水平的电离辐射中恢复的能力之间的可能联系。研究将包括：(1)对物种内和物种间的蛭体基因组选定区域进行测序和细胞遗传学研究，以获得蛭体基因组结构的总体描述；(2)描述蛭形体暴露于干燥和电离辐射后DNA损伤修复的基因组后果；(3) RNA干扰的发展，以确定DNA修复和重组基因在蛭形体抵抗干燥和电离辐射中的功能作用。这些实验旨在测试蛭形轮虫在其短暂的水生栖息地中频繁干燥导致的高水平DNA损伤中选择恢复的可能性，这种选择解释了它们基因组结构的基本特征，包括明显存在的同型染色体对，在其他标准组蛋白基因簇中缺乏组蛋白H2A基因，以及缺乏寄生的反转录转座子。这也可以解释它们在没有有性繁殖的情况下进化成功的原因。此外，考虑到蛭形基因组中一些已知的不寻常的特征，对这些生物进行持续的分子遗传学和比较研究可能会揭示出更多的新特征，这些知识可能对理解动物和植物中有性繁殖的优势、无性谱系的早期灭绝以及长期无性行为的基因组后果具有重要意义。尽管性是动植物繁殖的主要形式，但为什么会存在性，以及是什么原因导致放弃性的谱系灭绝，仍然是生物学中最令人困惑的问题之一。这个项目将调查无性谱系是进化死胡同这一规则的一个重大例外：蛭形纲的轮虫，这是一组水生无脊椎动物，其中的雄性，雌雄同体和性别都是未知的，然而，在数千万年的时间里，它们坚持并成功地进化和多样化成370多个物种。研究将探索蛭形无性与基因组结构之间的关系，以及蛭形在自然栖息地经受极度脱水的能力和最近发现的从极高水平辐射中恢复的能力之间的可能联系。来自哈佛大学和小型文理学院的本科生将参与该项目，并有机会在哈佛大学和伍兹霍尔海洋生物实验室进行研究。

Although there are many hypotheses, there is no general agreement regarding what maintains sexual reproduction and causes the early extinction of lineages that abandon it. A promising approach to the resolution of this fundamental problem is the study of rotifers of Class Bdelloidea, a highly unusual group of invertebrate animals comprising some 370 described species in which males, hermaphrodites, vestigial male structures, and meiosis are unknown and which has nevertheless persisted and successfully evolved and diversified for tens of millions of years. This project will investigate the relationship between bdelloid asexuality and genome structure, and its possible connection with the ability of bdelloids to withstand desiccation characteristic of their natural habitats and their recently discovered ability to recover from extremely high levels of ionizing radiation. Research will include: (1) Sequencing and cytogenetic studies of selected regions of bdelloid genomes within and between species in order to obtain an overall description of bdelloid genome structure; (2) Characterization of the genomic consequences of DNA damage repair following the exposure of bdelloids to desiccation and to ionizing radiation; and (3) Development of RNA interference to determine the functional role of DNA repair and recombination genes in bdelloid resistance to desiccation and to ionizing radiation. The experiments are designed to test the possibility that selection for recovery from high levels of DNA damage associated with the frequent desiccation of bdelloid rotifers in their ephemerally aquatic habitats accounts for basic features of their genome structure, including the apparent presence of syntenic chromosome pairs, the absence of genes for histone H2A in otherwise canonical histone gene clusters, and the lack of parasitic retrotransposons, and may also account for their evolutionary success in the absence of sexual reproduction. In addition, considering the several unusual features of bdelloid genomes already known, continued molecular genetic and comparative studies of these organisms may well reveal further novel features, knowledge of which may have important implications for understanding the predominance of sexual reproduction among animals and plants, the early extinction of asexual lineages, and the genomic consequences of prolonged asexuality. Although sex is the dominant form of reproduction among animals and plants, why sex persists, and what causes the extinction of lineages that abandon it, remains one of the most perplexing questions in biology. This project will investigate one of the great exceptions to the rule that asexual lineages are an evolutionary dead end: the rotifers of the Class Bdelloidea, a group of aquatic invertebrate animals in which males, hermaphrodites, and sex are unknown and which has nevertheless persisted and successfully evolved and diversified into more than 370 species over tens of millions of years. Research will explore the relationship between bdelloid asexuality and genome structure, and its possible connection with the ability of bdelloids to withstand the extreme dehydration they encounter in their natural habitat and their recently discovered ability to recover from extremely high levels of radiation. Undergraduate students from Harvard and small, liberal arts schools will participate in the project and will have the opportunity to carry out research at both Harvard and the Marine Biological Laboratory at Woods Hole.