Mapping the rDNA X-Y pairing site in Drosophila melanogaster males : test of rDNA structural region

Homologous chromosome pairing during early prophase I of meiosis is essential for subsequent meiotic events, including chromosome segregation. To understand how homologs pair, one important step is to find out where they pair. Is meiotic chromosome pairing sequence-specific? In Drosophila males, there is no crossingover and chiasma formation when chromosomes pair. Those phenomena simplify the analysis of pairing. Sex chromosome pairing in the males is mediated by ribosomal DNAs which are located in the X heterochromatin and near the base of the short arm of the Y. It has been shown that one complete copy of rDNA can stimulate X-Y pairing. Insertions of the 5' half of a rDNA gene including the promoter and upstream intergenic spacer at ectopic locations on the heterochromatically deficient X can restore X-Y pairing capacity. In this study we introduced a fragment containing most of the rDNA transcription unit but lacking the promoter and spacer sequences into embryos by P element-mediated transformation. Two X-linked transformants were identified by polytene chromosome in situ hybridization and genomic Southern blot analysis. We used two experimental approaches to investigate the function of this piece of rDNA in meiotic X-Y pairing. Both testis squashes and progeny testing to determine the X-Y non-disjunction rate give the same result: this piece of rDNA can not stimulate X-Y chromosome pairing during meiosis. This result together with previous studies demonstrates that the ribosomal DNA promoter region is the only region that contributes to X-Y chromosome pairing in D. melanogaster males and also implies that meiotic pairing may be localized in certain regions within a gene.