THE CENTRIC REGION OF THE X-CHROMOSOME RDNA FUNCTIONS IN MALE MEIOTICPAIRING IN DROSOPHILA-MELANOGASTER

In Drosophila melanogaster males, sex chromosome pairing at meiosis is ensured by so-called pairing site(s) located discretely in the centri c heterochromatin. The property of the pairing sites is not well under stood. Recently, an hypothesis has been proposed that 240 bp repeats i n the nontranscribed spacer region of rDNA function as the pairing sit es in male meiosis. However, considerable cytogenetic evidence exists that is contrary to this hypothesis. Hence, the question is whether th e chromosomal rDNA clusters, in which a high copy number of 240 bp rep eats exists, are involved in the pairing. In order to resolve the prob lem we X-rayed Drosophila carrying the X chromosome inversion ln(1)(SC SC8R)-S-V2L and generated free, mini-X chromosomes carrying a substant ial amount of rDNA. We defined cytogenetically the size of the mini-ch romosomes and studied their meiotic behavior. Our results demonstrate that the heterochromatin at the distal end of the inversion, whose len gth is approximately 0.4 times that of the fourth chromosome, includes a meiotic pairing site in the male. We discuss the cytological locati on of the pairing site and the possible role of rDNA in meiotic pairin g.