Mobile Genetic Elements in Sexual and Ancient Asexual Taxa

Marine Biological Laboratory, Woods Hole MA

Investigators

Abstract

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.