Evolutionary stability of the R1 retrotransposable element in the genus Drosophila

R1 is a non-long terminal repeat (non-LTR) retrotransposable element thatinserts into a specific sequence of insect 28S ribosomal RNA genes. We havepreviously shown that this element has been maintained through verticaltransmission in the melanogaster species subgroup of Drosophila. To addresswhether R1 elements have been vertically transmitted for longer periods ofevolutionary time, the analysis has been extended to 11 other species fromfour species groups of the genus Drosophila (melanogaster, obscura,testecea, and repleta). All sequenced elements appeared functional on thebasis of the preservation of their open-reading frames and consistentlyhigher rate of substitution at synonymous sites relative to replacementsites. The phylogenetic relationships of the R1 elements from all speciesanalyzed were congruent with the species phylogenies, suggesting that theR1 elements have been vertically transmitted since the inception of theDrosophila genus, an estimated 50-70 Mya. The stable maintenance of R1through the germ line appears to be the major mechanism for the widespreaddistribution of these elements in Drosophila. In two species, D.neotestecea of the testecea group and D. takahashii of the melanogastergroup, a second family of R1 elements was also present that differed insequence by 46% and 31%, respectively, from the family that was congruentwith the species phylogeny. These second families may represent occasionalhorizontal transfers or, alternatively, they could reflect the ability ofR1 elements to diverge into new families within a species and evolveindependently.