Retrotransposable elements R1 and R2 interrupt the rRNA genes of most insects.

A large number of insect species have been screened for the presence of the retrotransposable elements R1 and R2. These elements integrate independently at specific sites in the 28S rRNA genes. Genomic blots indicated that 43 of 47 insect species from nine orders contained insertions, ranging in frequency from a few percent to greater than 50% of the 28S genes. Sequence analysis of these insertions from 8 species revealed 22 elements, 21 of which corresponded to R1 or R2 elements. Surprisingly, many species appeared to contain highly divergent copies of R1 and R2 elements. For example, a parasitic wasp contained at least four families of R1 elements; the Japanese beetle contained at least five families of R2 elements. The presence of these retrotransposable elements throughout Insecta and the observation that single species can harbor divergent families within its rRNA-encoding DNA loci present interesting questions concerning the age of these elements and the possibility of cross-species transfer.