EVOLUTIONARY STABILITY OF THE R1 RETROTRANSPOSABLE ELEMENT IN THE GENUS DROSOPHILA

R1 is a non-long terminal repeat (non-LTR) retrotransposable element t hat inserts into a specific sequence of insect 28S ribosomal RNA genes . We have previously shown that this element has been maintained throu gh vertical transmission in the melanogaster species subgroup of Droso phila. To address whether R1 elements have been vertically transmitted for longer periods of evolutionary time, the analysis has been extend ed to 11 other species from four species groups of the genus Drosophil a (melanogaster, obscura, testecea, and repleta). All sequenced elemen ts appeared functional on the basis of the preservation of their open- reading frames and consistently higher rate of substitution at synonym ous sites relative to replacement sites. The phylogenetic relationship s of the R1 elements from all species analyzed were congruent with the species phylogenies, suggesting that the R1 elements have been vertic ally transmitted since the inception of the Drosophila genus, an estim ated 50-70 Mya. The stable maintenance of R1 through the germ line app ears to be the major mechanism for the widespread distribution of thes e elements in Drosophila. In two species, D. neotestecea of the testec ea group and D. takahashii of the melanogaster group, a second family of R1 elements was also present that differed in sequence by 46% and 3 1%, respectively, from the family that was congruent with the species phylogeny. These second families may represent occasional horizontal t ransfers or, alternatively, they could reflect the ability of R1 eleme nts to diverge into new families within a species and evolve independe ntly.