Turnover of R1 (type I) R2 (type II) retrotransposable elements in the ribosomal DNA of Drosophila melanogaster

• Published in: Genetics (Austin) Vol. 131; no. 1; pp. 129 - 142
• Main Authors: Jakubczak, J.L. (University of Rochester, Rochester, NY), Zenni, M.K, Woodruff, R.C, Eickbush, T.H
• Format: Journal Article
• Language: English
• Published: Bethesda, MD Genetics Soc America 01-05-1992; Genetics Society of America
• Subjects: ADN; Animals; ARN RIBOSOMAL; ARN RIBOSOMIAL; Autoradiography; Base Sequence; Biological and medical sciences; Biological Evolution; Classical genetics, quantitative genetics, hybrids; Cloning, Molecular; Crossing Over, Genetic; DNA Transposable Elements; DNA, Ribosomal - genetics; DROSOPHILA MELANOGASTER; Drosophila melanogaster - genetics; Fundamental and applied biological sciences. Psychology; GENE; Gene Conversion; GENES; GENETICA; Genetics; Genetics of eukaryotes. Biological and molecular evolution; GENETIQUE; Insects; Invertebrata; Investigations; Medical research; Molecular Sequence Data; MUTACION; MUTATION; NUCLEOTIDE; NUCLEOTIDOS; RIBOSOMAS; RIBOSOME; RNA, Ribosomal, 28S - genetics; VARIACION GENETICA; VARIATION GENETIQUE; X Chromosome; Retrotransposon; Copy number; Insecta; Ribosomal DNA; Transposable element; 28S-RNA; Drosophilidae; Molecular evolution; Transposition; Restriction fragment length polymorphism; Gene; Arthropoda; Invertebrata; Drosophila melanogaster; Diptera
• ISSN: 0016-6731; 1943-2631; 1943-2631
• DOI: 10.1093/genetics/131.1.129

R1 and R2 are distantly related non-long terminal repeat retrotransposable elements each of which inserts into a specific site in the 28S rRNA genes of most insects. We have analyzed aspects of R1 and R2 abundance and sequence variation in 27 geographical isolates of Drosophila melanogaster. The fraction of 28S rRNA genes containing these elements varied greatly between strains, 17-67% for R1 elements and 2-28% for R2 elements. The total percentage of the rDNA repeats inserted ranged from 32 to 77%. The fraction of the rDNA repeats that contained both of these elements suggested that R1 and R2 exhibit neither an inhibition of nor preference for insertion into a 28S gene already containing the other type of element. Based on the conservation of restriction sites in the elements of all strains, and sequence analysis of individual elements from three strains, nucleotide divergence is very low for R1 and R2 elements within or between strains ( 0.6%). This sequence uniformity is the expected result of the forces of concerted evolution (unequal crossovers and gene conversion) which act on the rRNA genes themselves. Evidence for the role of retrotransposition in the turnover of R1 and R2 was obtained by using naturally occurring 5' length polymorphisms of the elements as markers for independent transposition events. The pattern of these different length 5' truncations of R1 and R2 was found to be diverse and unique to most strains analyzed. Because recombination can only, with time, amplify or eliminate those length variants already present, the diversity found in each strain suggests that retrotransposition has played a critical role in maintaining these elements in the rDNA repeats of D. melanogaster