Phylogenetic relationships among the 12 sequenced Drosophila species and structures of R1 and R2 elements. (a) Phylogenetic relationships of the species based on maximum likelihood trees of their consensus 18S and 28S rRNA gene sequences. (b) Structures of the R1 and R2 elements found in each species. The 'A' and 'B' designations refer to the two divergent R1 lineages that are present among Drosophila species . Filled rectangles correspond to the 5' and 3' untranslated regions (UTRs). Open rectangles correspond to the open reading frames (ORFs). R1 elements have two overlapping ORFs in different frames. D. mojavensis contains R2 elements but a complete sequence could not be assembled. No trace of R2 elements could be identified in D. erecta, D. virilis and D. grimshawi.