Mary B. Comer

Extensive movement of LINES ONE sequences in beta-globin loci of Mus caroli and Mus domesticus.

LINES ONE (L1) is a family of movable DNA sequences found in mammals. To measure the rate of their movement, we have compared the positions of L1 elements within homologous genetic loci that are separated by known divergence times. Two models that predict different outcomes of this analysis have been proposed for the behavior of L1 sequences. (i) Previous theoretical studies of concerted evolution in L1 have indicated that the majority of the 100,000 extant L1 elements may have inserted as recently as within the last 3 million years. (ii) Gene conversion has been proposed as an alternative to a history of prolific recent insertions. To distinguish between these two models, we cloned and characterized two embryonic beta-globin haplotypes from Mus caroli and compared them with those of M. domesticus. In 9 of 10 instances, we observed an L1 element to be present in one chromosome and absent at the same site in a homologous chromosome. This frequency is quantitatively consistent with the known rate of concerted evolution. Therefore, we conclude that gene conversion is not required for concerted evolution of the L1 family in the mouse. Furthermore, we show that the extensive movement of L1 sequences contributes to restriction fragment length polymorphism. L1 insertions may be the predominant cause of restriction fragment length polymorphisms in closely related haplotypes.

The L1 Family of Repetitive Sequences in Mammals

Publisher Summary This chapter discusses the L1 family of repetitive sequences in mammals. All mammals that have been tested carry a family of large dispersed repetitive sequences in their genomes called “L1.” This element is a transposon that encodes proteins important for L1. The L1 family is different from other characterized transposons in a number of ways. L1 has no long terminal repeats. Most members of the family carry 5´ polar truncations of various lengths. These differences clearly suggest that L1 must replicate in a novel fashion. Arguments can be made both for and against a selfish mode of replication. The element transposes at a very high rate in the mouse and must generate a substantial genetic load for the species. This raises the possibility that Ll may play a noticeable role in the extinction of species.