L1 (LINE-1) retrotransposon evolution and amplification in recent human history

L1 (LINE-I) elements constitute a large family of mammalian retrotransposon s that have been replicating and evolving in mammals for more than 100 Myr and now compose 20% or more of the DNA of some mammals. Here, we investigat ed the evolutionary dynamics of the active human Ta L1 family and found tha t it arose similar to 4 MYA and subsequently differentiated into two major subfamilies, Ta-0 and Ta-l, each of which contain additional subsets. Ta-l, which has not heretofore been described, is younger than Ta-0 and now acco unts for at least 50% of the Ta family. Although Ta-0 contains some active elements, the Ta-l subfamily has replaced it as the replicatively dominant subfamily in humans; 69% of the loci that contain Ta-l inserts are polymorp hic for the presence or absence of the insert in human populations, as comp ared with 29% of the loci that contain Ta-0 inserts. This value is 90% for loci that contain Ta-ld inserts, which are the youngest subset of Ta-l and now account for about two thirds of the Ta-l subfamily. The successive emer gence and amplification of distinct Ta L1 subfamilies shows that L1 evoluti on has been as active in recent human history as it has been found to be fo r rodent L1 families. In addition, Ta-l elements have been accumulating in humans at about the same rate per generation as recently evolved active rod ent L1 subfamilies.