Constitutive magnification by the Ybb- chromosome of Drosophila melanogaster
1993
Komma, D.J. | Glass, S.J. | Endow, S.A.
Y(bb-) is an rDNA-deficient chromosome of Drosophila that has often been 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 chromosome causes low levels of bb magnification in bb+ flies. We refer to the ability of bypass the rDNA deficiency requirement for magnification as 'constitutive' magnification. We have magnified the ribosomal genes on the Y(bb-) chromosome and analysed the revertant chromosomes using genetic and molecular methods. We find that magnified Y(bb-) chromosomes also exhibit constitutive magnifier activity. Molecular analysis shows that both type 1 and type 2 intron+ ribosomal gene repeats are associated with magnified Y(bb-) chromosomes. Type 2 introns have been described previously in the rDNA 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 present in tandem arrays. The constitutive magnifier activity of Y(bb-) may reside either in the structurally unusual ribosomal gene intervening sequences associated with the chromosome, or in the locus on Y(L) that is required for magnification to occur.
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