Rapid amplification of a retrotransposon subfamily is evolving the mouse genome

Abstract

Retrotransposition affects genome structure by increasing repetition and producing insertional mutations^1,2. Dispersion of the retrotransposon L1 throughout mammalian genomes suggests that L1 activity might be an important evolutionary force¹. Here we report that L1 retrotransposition contributes to rapid genome evolution in the mouse, because a number of L1 sequences from the T_F subfamily are retrotransposition competent. We show that the T_F subfamily is large, young and expanding, containing approximately 4,800 full-length members in strain 129. Eleven randomly isolated, full-length T_F elements averaged 99.8% sequence identity to each other, and seven of these retrotransposed in cultured cells. Thus, we estimate that the mouse genome contains approximately 3,000 active T_F elements, 75 times the estimated number of active human L1s. Moreover, as T_F elements are polymorphic among closely related mice, they have retrotransposed recently, implying rapid amplification of the subfamily to yield genomes with different patterns of interspersed repetition. Our data show that mice and humans differ considerably in the number of active L1s, and probably differ in the contribution of retrotransposition to ongoing sequence evolution.