Mobile Genetic Elements in Sexual and Ancient Asexual Taxa

Harvard University, Cambridge MA

Investigators

Abstract

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.