Mobile elements: drivers of genome evolution

Mobile elements within genomes have driven genome evolution in diverse ways. Particularly in plants and mammals, retrotransposons have accumulated to constitute a large fraction of the genome and have shaped both genes and the entire genome. Although the host can often control their numbers, massive expansions of retrotrans-posons have been tolerated during evolution. Now mobile elements are becoming useful tools for learning more about genome evolution and gene function. Mobile, or transposable, elements are preva-lent in the genomes of all plants and animals. Indeed, in mammals they and their recogniz-able remnants account for nearly half of the genome (1, 2), and in some plants they con-stitute up to 90 % of the genome (3). If, as many believe, the origins of life are in an “RNA world ” followed by reverse transcrip-tion into DNA, then mobile elements coul