Evolutionary stability of the R1 retrotransposable element in the genus Drosophila

R1 is a non-long terminal repeat (non-LTR) retrotransposable element that inserts into a specific sequence of insect 28S ribosomal RNA genes. We have previously shown that this element has been maintained through vertical transmission in the melanogaster species subgroup of Drosophila. To address whether R1 elements have been vertically transmitted for longer periods of evolutionary time, the analysis has been extended to 11 other species from four species groups of the genus Drosophila (melanogaster, obscura, testecea, and repleta). All sequenced elements appeared functional on the basis of the preservation of their open-reading frames and consistently higher rate of substitution at synonymous sites relative to replacement sites. The phylogenetic relationships of the R1 elements from all species analyzed were congruent with the species phylogenies, suggesting that the R1 elements have been vertically transmitted since the inception of the Drosophila genus, an estimated 50-70 Mya. The stable maintenance of R1 through the germ line appears to be the major mechanism for the widespread distribution of these elements in Drosophila. In two species, D. neotestecea of the testecea group and D. takahashii of the melanogaster group, a second family of R1 elements was also present that differed in sequence by 46% and 31%, respectively, from the family that was congruent with the species phylogeny. These second families may represent occasional horizontal transfers or, alternatively, they could reflect the ability of R1 elements to diverge into new families within a species and evolve independently.