Mobile Genetic Elements in Sexual and Ancient Asexual Taxa

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Abstract The goal of this research is to determine the generality and significance of the finding that rotifers of the Class Bdelloidea, in striking contrast to other classes of the Phylum Rotifera and all other eukaryotes that have been examined, appear not to contain retrotransposons. Confirmation of the absence of retrotransposons and, possibly, of DNA transposons as well, would represent an extraordinary departure from the universal occurrence of such elements in all other eukaryotes and would therefore have the potential of providing new insights into the evolution, maintenance, and role of mobile genetic elements. Moreover, such a finding would be of fundamental interest because of another highly unusual and possibly related attribute of bdelloid rotifers, their apparent asexuality. Two complementary experimental approaches will be followed in the proposed research. The first utilizes polymerase chain reaction (PCR) with nested sets of degenerate primers designed to amplify specific conserved domains that are diagnostic for each of the major classes of RNA and DNA transposons. Resulting amplicons of the expected size will be cloned and sequenced to test their identity. Such screening will include diverse species of bdelloid rotifers and, as controls, diverse monogonont rotifers and certain other animal phyla not previously examined for such elements. In the second approach to be followed, approximately one megabase of bdelloid DNA from cosmid libraries will be sequenced in order to identify mobile elements for which no effective PCR primers can be designed, including ancient inactive elements that may have diverged beyond the detection capability of PCR and other solution hybridization methods. Contrary to the generalization that the loss of sexual reproduction is a dead end in evolution, the entire Class Bdelloidea appears to have evolved for tens of millions of years without males, meiosis, or any form of sexual reproduction. A finding that bdelloids lack RNA and/or DNA transposons would therefore suggest that such elements are not retained in asexual lines over evolutionary time periods and/or that their loss or inactivation was a pre-condition for the success of bdelloids in avoiding extinction and undergoing extensive evolution without sexual reproduction. Thus, investigation of the status of mobile elements in this novel system may also reveal fundamental relationships between the presence of such elements and the evolution and maintenance of sexual reproduction.