AMPLIFICATION OF AN ANCESTRAL MAMMALIAN L1 FAMILY OF LONG INTERSPERSED REPEATED DNA OCCURRED JUST BEFORE THE MURINE RADIATION

ABSTRACT Each mammalian genus examined so far contains 50,000-100,000 members of an Li (LINE 1) family of long interspersed repeated DNA elements. Current knowledge on the evolution of LI families presents a paradox because, although Li families have been in mammalian genomes since before the mammalian radiation -80 million years ago, most members of the Li families are only a few million years old. Accordingly it has been suggested either that the extensive amplification that characterizes present-day Li families did not occur in the past or that old members were removed as new ones were generated. However, we show here that an ancestral rodent Li family was extensively amplified '10 million years ago and that the relics (""60,000 copies) of this amplification have persisted in modern murine genomes (Old World rats and mice). This amplification occurred just before the divergence of modern murine genera from their common ancestor and identifies the murine node in the lineage of modern muroid rodents. Our results suggest that repeated amplification of LI elements is a feature of the evolution of mammalian genomes and that ancestral amplification events could provide a useful tool for determining mammalian lineages.