Regulation of rRNA gene number in Drosophila melanogaster: new aspects resulting from the use of free duplications.

We have isolated a bobbed (bb) mutant on the free duplication Dp(1;f)122bb(+) and we have measured the rDNA content of the bb(+) and the bb loci in genetic combinations in which none of the phenomena involved in the change of the rDNA redundancy occurs. We have also measured the rDNA content of the two bb loci carried by the free duplications in two different genetic combinations: (1) XXNO(-)/Dp122bb(+) and XXNO(-)/Dp122bb females in which there are two attached X chromosomes completely deleted for the nucleolus organizer (NO) regions and therefore the only rDNA is contributed by the free duplication; (2) X/Dp122bb(+) and X/Dp122bb males, in which there are two bb loci, one on the X chromosome and the other on the X free duplication. The bb(+) and the bb duplications produced an overall increase of the rDNA content in the two genetic conditions tested. These results are not in favour of both a cis and trans effect of the regulator locus (cr(+) locus) hypothesised as being involved in the disproportionate replication of rRNA genes.