RI AND R2 RETROTRANSPOSABLE ELEMENTS OF DROSOPHILA EVOLVE AT RATES SIMILAR TO THOSE OF NUCLEAR GENES

The non-long-terminal repeat retrotransposable elements, R1 and R2, in sert at unique locations in the 283 ribosomal RNA genes of insects. Ba sed on the nucleotide sequences of these elements in the eight members of the melanogaster species subgroup of the genus Drosophila, they ha ve been maintained by vertical germline transmission for the 17 - 20 m illion year history of this subgroup. The stable inheritance of R1 and R2 within these species has enabled a determination of their nucleoti de substitution rates. The sequence of the R1 and R2 elements from D. ambigua, a member of the obscure species group, has also been determin ed to enable an extrapolation of this rate over an estimated 45 - 60 m illion years. The mean rate of substitutions at synonymous sites (K-s) was 6.6 and 9.6 times the rate at replacement sites (K-a) in the R1 a nd R2 elements, respectively. Both elements appear to have been under selective pressure to maintain their open reading frames and thus thei r ability to retrotranspose for most of their evolution in these linea ges. Using the rate of change at synonymous sites (K-s) as the best in dicator of the nucleotide substitution rate, the mean K-s values for R 1 and R2 were 2.3 and 2.2 times that of the alcohol dehydrogenase (Adh ) genes. However, this faster rate is a result of the lower codon usag e bias of R1 and R2 compared with that of Adh. When the K-s rates of R 1 and R2 were compared with that of a larger number of nuclear genes a vailable from at least two of the nine species under investigation, R1 and R2 were found to evolve in most lineages at rates similar to that of nuclear genes with low codon bias. The ability of R1 and R2 to mai ntain their presence in this species subgroup by retrotransposition wh ile exhibiting rates of nucleotide evolution similar to nuclear genes suggests these transposition events are rare or not as error prone as that of retroviruses.