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.
