Evolution of R1 and R2 in the rDNA units of the genus Drosophila

R1 and R2 are non-long terminal repeat (non-LTR) retrotransposable elements that specifically insert in the 28S ribosomal RNA (rRNA) genes of insects. Using the Drosophila genus, which includes some of the best characterized insect taxa, we have conducted a number of studies on the evolution of these elements. We find that R1 and R2 are subject to the same recombinational forces that give rise to the concerted evolution of the rDNA units. The turnover of R1 and R2 elements can be readily documented in different strains of D. melanogaster using 5' truncated elements as restriction-length polymorphisms. This turnover leads to uniform populations of elements with nucleotide sequence divergence of different copies averaging only 0.23% for the R2 and 0.47% for the R1 elements. Molecular phylogenetic analysis of elements from 16 different species of Drosophila suggests that these elements have been stable components of the rDNA locus for the 50-70 million year history of the Drosophila genus. Using changes at synonymous positions within the protein-encoding regions as estimates of the baseline substitution rate, it could be shown that R1 and R2 are evolving at rates similar to that of typical protein encoding genes provided corrections are made for the low codon bias of the elements. R1 and R2 are clearly well-adapted for their existence in the rDNA units of their host.