A single lineage of r2 retrotransposable elements is an active, evolutionarily stable component of the Drosophila rDNA locus

R2 elements are non-long-terminal-repeat (non-LTR) retrotransposons that insert specifically in the 28S rRNA genes of many insects. Previous reports concerning this element in the genus Drosophila have suggested that R2 elements are absent from many species of this genus, particularly those species from the subgenus Drosophila. In this report, we present an extensive study of the distribution and evolution of R2 elements in Drosophila. A PCR survey of 59 species from 23 species groups of the two major Drosophila subgenera found that R2 elements are present in all but two species of the melanogaster species subgroup. Phylogenetic analysis based on partial nucleotide sequences of R2 elements from 23 species demonstrates that the relationships of R2 elements are congruent with those of the Drosophila species phylogeny, suggesting that these elements have been vertically inherited since the divergence of this genus some 60 MYA. Sequence variation between different copies of R2 elements within each species was less than 0.16%, indicating that these elements are undergoing concerted evolution similar to that of the 28S genes. Several properties of the R2 sequences suggest that these elements depend on retrotransposition in addition to simple recombination to remain within the rDNA locus: the rates of synonymous substitutions averaged 4.8 times the rate of replacement substitutions, 82 of 83 R2 copies partially sequenced contained intact open reading frames, and, finally, length variation associated with the poly(A) 3' tails indicated that many R2 copies are the direct result of retrotransposition.