The cytogenetic boundaries of the rDNA region within heterochromatin of theXchromosome ofDrosophila melanogasterand their relation to male meiotic pairing sites

Abstract

Summary: The proximal breakpoints of the inversion chromosomesIn(1)ω^m4andIn(1)^m51bwere shown, byin situhybridization, to define the boundaries of the ribosomal DNA region located within theXchromosome heterochromatin (Xh). We estimate thatat least95% of the rDNA is located between theIn(1)ω^m4andIn(1)ω^m51bproximal breakpoints. In contrast only 60–70% of the Type I intervening sequences located inXhare located between these breakpoints. The Type I intervening sequences in the rDNA region occur as inserts in the 28S rRNA sequences while the remainder of the sequences are distal to theIn(1)ω^m4breakpoint and not associated with rRNA genes.The regions of Xh which contain rDNA and Type I intervening sequences were related to regions shown by Cooper (1964) to contribute to meiotic pairing between theXandYchromosomes in male Drosophila. We demonstrate that the rRNA coding region contributes toX/Ypairing. However, no single region of Xh is required for fidelity of male meiotic pairing of the sex chromosomes.