Jumping Genes Reveal Kangaroos' Origins

viruses do, retroposed elements are depos- ited in other parts of the same genome in the same cell—including in the germ line cells—cutting a gap in a DNA strand and inserting themselves there. These copies remain in their new locus. It is extremely rare that a retroposed element is cleanly excised sometime after insertion. Afer millions of years, hundreds of thousands of them are now littered throughout marsupials' genomes. Also, the way they spread through the genome means they can occur in idiosyn- cratic patterns. Jumping genes are so widespread in marsupial genomes that when a copy jumps, it often lands in the middle of an older jumping gene, creating one retroposon nested within another one. Retroposons, and especially nested ones, are unlikely to arise independently in another species in exactly the same part of the genome by chance. So if different species share a few of the same nested retroposed elements, chances are over- whelming that they all got them from a long-lost ancestor. In two marsupial genomes that were recently sequenced, Nilsson, Schmitz, and colleagues identified thousands of these nested retroposed elements—more than 8,000 in the genome of the South