ISRAEL JOURNAL OF ECOLOGY & EVOLUTION, Vol. 52, 2006, pp. ???–??? *Author to whom correspondence should be addressed. E-mail: alan.schulman@helsinki.i Accepted ??? 200? RetRotRansposons: metapaRasites and agents of genome evolution FRANçOIS SAbOT, a RUSLAN KALENdAR, a MARKO JääSKELäINEN, a ChANG WEI, a JAAKKO TANSKANEN, a,b ANd ALAN h. SChULMAN a,b, * a MTT/BI Plant Genomics Laboratory, Insitute of Biotechnology, Viikki Biocenter, University of Helsinki, P.O. Box 56, Viikinkaari 4, FIN-00014 Helsinki, Finland b Plant Genomics, Biotechnology and Food Research, MTT Agrifood Research Finland, Myllytie 10, FIN-31600 Jokioinen, Finland AbSTRACT Transposable elements comprise the bulk of higher plant genomes. The majority of these elements are the Class I LTR retrotransposons, which trans- pose via an RNA intermediate in a “Copy-and-Paste” mechanism. because retrotransposons use cellular resources and their own enzymes to replicate independently of the genome as a whole, and have thereby become in many cases more predominant than the cellular genes, they have been considered “selish DNA” and nuclear parasites. They are thought to share many features of the internal life cycle of retroviruses such as hIV (lentiviruses). however, whereas at least some of the retroviruses arriving in an organism during an in- fection must be functional in order for the infection to proceed, some LTR ret- rotransposon families appear to completely lack active members even though they remain mobile. Furthermore, the process of retrotransposition is inher- ently error-prone and mutagenic, giving rise to “pseudospecies,” or clusters of imperfect copies. The non-autonomous retrotransposons are able to cis- and trans-parasitize host retrotransposons to gain mobility, much as do defective interfering particles of RNA viruses. hence, a complex dynamic is set up, whereby the impact of retrotransposons on genomes can be under selection on the organismal level; the impact of non-autonomous retrotransposons on autonomous ones can likewise be under selection if there is selection on the autonomous elements themselves. We are exploring the retrotransposon life cycle and the causes and possible consequences of non-autonomy at each stage regarding genome evolution. Keywords: LTR retrotransposon, genome evolution, parasitism, non-autono- my, life cycle, insertional mutagenesis INTroDucTIoN Transposable elements, including both class II or DNA transposons and class I or retrotransposons, comprise the bulk of higher plant genomes, ranging from 15% of the