Expression of rDNA insertions during rDNA magnification in D. melanogaster

In D. melanogaster, many ribosomal genes contain an intervening sequence that interrupts the 28S rRNA gene. In wild-type flies, type I and type II insertions are rarely transcribed and the transcripts of the interrupted genetic units do not contribute significantly to the production of mature ribosomal RNA. We demonstrate that during rDNA magnification, transcription of short type I sequences and of type II sequences is respectively, about 4- and about 16-fold that observed in unmagnified homozygous bobbed females used as control. In subsequent generation (bbm1) we have observed, for transcription of short type I sequences, that this elevated level is maintained and, for transcription of type II sequences, that the level reverts to one comparable with the control. Transcription of both insertion sequences decreases in subsequent generations (bbm19), reaching a level comparable with the controls.