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

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Abstract
Summary: The proximal breakpoints of the inversion chromosomesIn(1)ωm4andIn(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)ωm4andIn(1)ωm51bproximal 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)ωm4breakpoint 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.