Multiple sex chromosomes in Drosophila miranda: a system to study the degeneration of a chromosome.

Drosophila miranda possesses an intriguing sex chromosome constitution. While female metaphase plates have 10 chromosomes (diploid set), in males only 9 chromosomes can be identified. The missing homologue has been translocated to the Y, forming a neo-Y chromosome which is polytenized in the salivary gland cells. This report presents a detailed characterization of DNA, isolated from D. miranda flies. In situ hybridizations, using cRNA transcribed from unfractionated D. miranda DNA, reveal hybridization to the neo-Y with label distributed over the entire chromosome. The original partner of the translocated chromosome, X2, is essentially unlabelled. These results suggest that repetitive DNA sequences "invade" the translocated chromosome. This result is discussed with reference to the hypothesis of "degeneration" of the Y chromosome, formulated by Muller (1918, 1932a).