MOSAIC EVOLUTION OF RODENT B1 ELEMENTS

We have determined sequences of PCRamplified B1 elements from hamster and rat (Myomorpha), chipmunk (Sciuromorpha), and guinea pig (Caviomor pha). 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 sep arate rodent lineages. We exclude the possibility of convergent mutati ons to explain certain diagnostic characters within the modern B1 quas i-dimers and view these elements as mosaic structures assembling preex isting mutations. Furthermore, the presence of Alu-like structural mot ifs supports the hypothesis of the monophyletic origin of Alu and B1 r epeats, i.e., from a common 7SL RNA-derived retroposing monomeric elem ent.