Incomplete Y chromosomes promote magnification in male and female Drosophila.

We have recently shown that magnification, an increase in the number of ribosomal RNA genes (rDNA) in gametes produced by rDNA-deficient flies, can occur in female Drosophila if they have a Y chromosome. We now have tested several X-Y translocation and recombinant chromosomes to determine which parts of the Y chromosome are necessary for magnification to occur in females. Our data indicate that the required region is the distal part of the long arm of the Y chromosome, YL. We have also used X-Y translocation chromosomes to study magnification of rDNA-deficient X chromosomes in males. Our data show that the region of the Y chromosome from the distal end of the nucleolus organizer through the centromere is not required for magnification in males. The frequency of magnification in males with rDNA-deficient Y fragments is comparable to that produced by Ybb-, a chromosome that has often been used to produce magnification in males. These results demonstrate that the Ybb-chromosome is not uniquely effective in causing magnification to occur in males. The results of these studies imply that sequences present on YL are required for magnification to occur in females; these sequences are probably also required for magnification in males. Since unequal sister chromatid exchange has been implicated as the major mechanism of ribosomal gene increase during magnification, the YL sequences required for magnification may be involved in encoding or regulating products needed for sister chromatid recombination in germ-line cells.