Molecular Biology and Evolution, Vol 10, 163-185, Copyright © 1993 by Society for Molecular Biology and Evolution
WD Burke, DG Eickbush, Y Xiong, J Jakubczak and TH Eickbush
R1 and R2 are retrotransposable elements that integrate at specific sites
in the 28S ribosomal RNA (rRNA) genes of Bombyx mori and Drosophila
melanogaster. We have previously shown that most insect species contain
insertions in their 28S genes at the R1 and/or R2 site. We have sequenced
the 3' half of R1 and R2 elements from three additional insect species: the
fungus gnat, Sciara coprophila (Diptera); the Japanese beetle, Popillia
japonica (Colleoptera); and the parasitic wasp, Nasonia vitripennis
(Hymenoptera). The elements were obtained by screening lambda phage genomic
clones containing rDNA units and by a polymerase chain reaction approach
using degenerate primers to conserved sequences in the
reverse-transcriptase domain, in combination with a second primer to the
28S gene 3' of the insertion site. Comparisons of the sequences of R1 and
R2 from four insect orders suggest that the organization of their
open-reading frames has been conserved and is therefore likely to be
similar throughout insects. This sequence analysis also indicates that,
except for 5' truncations generated during the retrotransposition process
itself, most elements have not accumulated mutations that would make them
inactive. Popillia japonica and N. vitripennis differed from previously
described species, in that (a) P. japonica contained multiple families of
R2 and (b) N. vitripennis contained multiple families of R1. Nucleotide
sequence identity between these different families is low. Amino acid
sequence identity of their open-reading frames averaged only 41% for the R2
families of P. japonica and 35% for the R1 families of N. vitripennis. The
presence of multiple highly divergent families of elements within a species
suggests either that each insertion family is able to maintain its copy
number without eliminating the other families in its competition for a
limited number of 28S genes or that there has been extensive horizontal
transfer of R1 and R2 elements between insect species.
ORIGINAL ARTICLE
Sequence relationship of retrotransposable elements R1 and R2 within and between divergent insect species
Department of Biology, University of Rochester, New York 14627.
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