Mobile DNA: genomes under the influence

Mobile DNA is a relatively loose term that encompasses an amazing diversity of genetic elements that are capable of movement from one genomic locale to another, and can often invade other genomes. Transposable elements (transposons), among the most widespread forms of mobile DNA, populate the genomes of most living organisms and have propagated to enormous numbers in many eukaryotes (for example, about half of the human genome is directly derived from transposable elements). An understanding of the behavior of transposable elements is therefore essential to our understanding of how genomes function and evolve. A recent conference on mobile DNA provided many outstanding examples of research in this rich and vibrant field, a few of which are highlighted here. We focus on work that advances our understanding of the impact of transposable elements on the evolutionary trajectories of their host genomes.With the advent of genomics, the significant influence of transposable elements in shaping the genomes of virtually all organisms is becoming fully appreciated. Most of the transposable elements in mammalian genomes are retro-transposons, transposable elements that transpose via an RNA intermediate. More than a million copies of the Alu retrotransposon occur scattered throughout the human genome. Mark Batzer (Louisiana State University, Baton Rouge, USA) reported work showing that nearly 500 events of non-reciprocal recombination between these interspersed Alu elements have removed around 400 kb of human genomic DNA since the divergence of the human and chimpanzee lineages. Prescott Deininger (Tulane University, New Orleans, USA) speculated that the trigger for these recombination events could be the enzymatic machinery encoded by a few active long interspersed nuclear elements (LINE-1 elements) remaining in the genome. Indeed, Deininger reported the stunning observation that the endonuclease activity encoded by a single active LINE-1 can create hundreds of DNA double-stra