Mosaic evolution of rodent B1 elements.

We have determined sequences of PCR-amplified B1 elements from hamster and rat (Myomorpha), chipmunk (Sciuromorpha), and guinea pig (Caviomorpha). Between three and six B1 subfamilies were found in these species. In the phylogenetic analysis B1 sequences of hamster, mouse, and rat clustered separately from those of chipmunk and those of guinea pig. This is consistent with an independent evolution of B1 elements in separate rodent lineages. We exclude the possibility of convergent mutations to explain certain diagnostic characters within the modern B1 quasi-dimers and view these elements as mosaic structures assembling preexisting mutations. Furthermore, the presence of Alu-like structural motifs supports the hypothesis of the monophyletic origin of Alu and B1 repeats, i.e., from a common 7SL RNA-derived retroposing monomeric element.