The effects of a promoter-minus mutation on meiotic pairing activity of rDNA IGS repeats in Drosophila males

In Drosophila melanogaster males, sex chromosome pairing during meiosis is mediated by rRNA genes which are located in the heterochromatin of the X chromosome. Analysis of constructed rDNA fragments showed that promoter-containing rDNA fragments are autonomously capable of being transcribed and of functioning as X-Y pairing sites. Pairing ability is proportional to the dose of 240bp intergenic spacer (IGS) repeats. Each of these repeats contains a functional rDNA promoter. In this study, we made point mutations within the promoter of the 240bp IGS repeat, ligated multiple repeats containing these mutations head-to-tail and then introduced a resulting mutant oligomer onto an rDNA- deficient X chromosome by P-element mediated transformation. The effects of these transgene insertions on the frequency of X-Y pairing and disjunction were analyzed both by cytological and genetic experiments. The failure of these sequences to mediate X-Y pairing and normal disjunction bears on the importance of transcriptional activity for the pairing function of ribosomal repeats.