AMPLIFICATION OF THE ANCIENT MURINE LX FAMILY OF LONG INTERSPERSED REPEATED DNA OCCURRED DURING THE MURINE RADIATION

We identified and characterized the relies of an ancient rodent L1 fam ily, referred to as Lx, which was extensively amplified at the time of the murine radiation about 12 million years ago, and which we showed was ancestral to the modern L1 families in rat and mouse. Here we have extended our analysis of the Lx amplification by examining more murin e and nonmurine species for Lx sequences using both blot hybridization and the polymerase chain reaction for a total of 36 species. In addit ion we have determined the relative copy number and sequence divergenc e, or age, of Lx elements in representative murine genera. Our results show that while Lx sequences are confined to murine genera, the exten t of the amplification was different in the different murine lineages, indicating that the amplification of Lx did not precede, but was coin cident with, the murine radiation. The implications of our findings fo r the evolutionary dynamics of L1 families and the utility of ancestra l amplification events for systematics are discussed.