CONSTITUTIVE MAGNIFICATION BY THE Y-BB-CHROMOSOME OF DROSOPHILA-MELANOGASTER

Y-bb- is an rDNA-deficient chromosome of Drosophila that has often bee n used in magnification experiments to induce high-frequency reversion of bobbed (bb) chromosomes. We observed previously that Y-bb- causes ring chromosome loss even when the rings are bb(+), suggesting that Y- bb- induces magnifying sister chromatid exchanges in bb(+) rings. Here we show that the Y-bb- chromosome causes low levels of bb magnificati on in bb(+) flies. We refer to the ability of Y-bb- to bypass the rDNA deficiency requirement for magnification as 'constitutive'' magnifica tion. We have magnified the ribosomal genes on the Y-bb- chromosome an d analysed the revertant chromosomes using genetic and molecular metho ds. We find that magnified Y-bb- chromosomes also exhibit constitutive magnifier activity. Molecular analysis shows that both type 1 and typ e 2 intron(+) ribosomal gene repeats are associated with magnified Y-b b- chromosomes. Type 2 introns have been described previously in the r DNA of both X and Y chromosomes. However, type 1 intervening sequences are thought to be present only in X, but not Y, ribosomal genes. Some of the Y-bb- type 1 insertions differ from those present in the rDNA of X chromosomes in that they contain an EcoRI site, and some may be p resent in tandem arrays. The constitutive magnifier activity of Y-bb- may reside either in the structurally unusual ribosomal gene interveni ng sequences associated with the chromosome, or in the locus on Y-L th at is required for magnification to occur.