We have examined rDNA magnification in Drosophila melanogaster males carrying one of 11 recombination-or repair-defective mutations representing seven loci. We show that mutations defined by a defect in postreplication repair (mus-101, mei-41, and mus-108) are also defective in rDNA magnification, whereas mutations that do not affect postreplication repair have little or no effect on magnification.mei-41 inhibits only premeiotic magnification events, while mus-108 blocks both premeiotic and meiotic events. This suggests that meiotic and premeiotic events share some but not all functions. A molecular analysis of rDNA magnification reveals that in mus-108 males, changes in the rDNA restriction pattern can occur within one or a few generations under magnifying conditions. We interpret these data in terms of the role ofDNA repair systems in rDNA magnification and in terms of stable maintenance of tandemly repeated genes.In Drosophila males there are two clusters of tandemly repeated rRNA genes (rDNA), each with -250 copies. One of these arrays is located in the proximal heterochromatin of the X chromosome, and the other, on the Y chromosome (1, 2). Each array comprises a number of distinct classes of rRNA genes that differ by the presence or absence of transposon-like insertions and by variations in the spacer length (3, 4). Different wild-type strains of D. melanogaster show extraordinary differences in the degree to which various repeat classes are represented in their rDNA (5, 6). Individuals with partial deficiencies at either cluster, known as bobbed (bb) mutants, are also found in wild-type populations and in laboratory stock collections (1). However, within individual laboratory stocks, changes in rDNA redundancy or repeat class composition are rare (5). This suggests that there are events or processes that can change the copy number of the rDNA or the representation of various repeat classes.Alterations in X chromosomal rDNA redundancy occur in males carrying a Ybb-chromosome or its derivatives (7-9). Ybb-is a Y chromosome from which >80% of the rDNA is deleted (8). These changes in rDNA redundancy may be observed as either stable reversions (magnification) from bb to bb+ or as mutations (reduction) from bb+ to bb or from bb to bbl (bobbed lethal). Several lines of evidence demonstrate that magnification can occur both meiotically and premeiotically (8, 9). Considerable evidence suggests that meiotic magnification and reduction are reciprocal results of unequal sister-chromatid exchange occurring within the X chromosome rDNA (8-10).We have examined rDNA magnification in males carrying repair-and/or recombination-defective mutations (11-13) at one of seven X chromosomal loci. Previous experiments showed that two alleles of mei41 strongly inhibit magnification (14). We now have extended those observations by further testing one of these alleles and by demonstrating a magnification defect for three other alleles of mei-4. Alleles of two other loci required for postreplication repair, mus-101 and mus-108...