R1-elements and R2-elements are rarely transferred between the rRNA loci of the X and Y chromosomes (bb and Ybb respectively). Retrotransposition of R1-elements occurs predominantly in the male germline, while retrotransposition of R2-elements is more evenly divided between the germlines of both sexes. The rate of elimination of R1-elements and R2-elements from the Ybb array on the Y chromosome is twice that of the rate of elimination from the bb array on the X chromosome. Both the bb and Ybb loci contain recombinational "hotspots" where R1-elements and R2-elements are lost at a rate eight times faster than elements located outside of the hotspots. Most R1-element and R2-element eliminations appear to occur by large intrachromosomal events that involve multiple rDNA units.
Elements R1 and R2 are subject to the same recombinational forces that give rise to the concerted evolution of the rDNA units. Molecular phylogenetic analysis of elements from 16 Drosophila species suggests these elements have been stable components of the rDNA locus for the 50-70 million year history of the Drosophila genus.
Analysis of 5' junction sequences of 94 R2-elements from 14 species reveals that no sequences at the 5' end of the R2-element appear to be required for element integration. A model in which the R2-element reverse transcriptase is capable of switching templates from the R2-element RNA transcript to the upstream 28SrRNA gene can best explain the observed 5' junction sequences.
R1-elements and R2-elements are rarely transferred between the rRNA loci of the X and Y chromosomes (bb and Ybb respectively). Retrotransposition of R1-elements occurs predominantly in the male germline, while retrotransposition of R2-elements is more evenly divided between the germlines of both sexes. The rate of elimination of R1-elements and R2-elements from the Ybb array on the Y chromosome is twice that of the rate of elimination from the bb array on the X chromosome. Both the bb and Ybb loci contain recombinational "hotspots" where R1-elements and R2-elements are lost at a rate eight times faster than elements located outside of the hotspots. Most R1-element and R2-element eliminations appear to occur by large intrachromosomal events that involve multiple rDNA units.
Elements R1 and R2 are subject to the same recombinational forces that give rise to the concerted evolution of the rDNA units. Molecular phylogenetic analysis of elements from 16 Drosophila species suggests these elements have been stable components of the rDNA locus for the 50-70 million year history of the Drosophila genus.
Analysis of 5' junction sequences of 94 R2-elements from 14 species reveals that no sequences at the 5' end of the R2-element appear to be required for element integration. A model in which the R2-element reverse transcriptase is capable of switching templates from the R2-element RNA transcript to the upstream 28SrRNA gene can best explain the observed 5' junction sequences.
Aspects of R2-element abundance and sequence variation in 27 geographical isolates of D.melanogaster have been analysed.