Dosage compensation for a chromosome that comprises an X chromosome and an autosome as its two homologs in interspecific hybrids of Drosophila miranda and D. persimilis.

A chromosomal element (C) of an interspecific hybrid of D. miranda (females) and D. persimilis (males) represented a unique situation, where between its two homologous, one derived from an X chromosome (X2 of miranda) and the other from an autosome (3rd chromosome of persimilis). In cytological preparations of polytene nuclei, the X chromosomal homolog in hybrid males exhibited the male-X like inflated structure, known as prerequisite for hypertranscription; whereas the autosomal homolog existed as an haploid autosomal arm. In hybrid females, both the homologs had similar diameter and stainability. This difference in cytomorphology of the X2 homolog between two sexes made the C element potential to transcribe higher in males than in females, raising the inevitable question of compensation. Cellular autoradiography, using 3H-UR, was employed to measure the total transcription of the C element (X2 + 3rd) in hybrid sexes. Results revealed that, although the X2-homolog was hyperactive in males, the total transcription of the C element was equal (relative to autosomal transcription) in both hybrid sexes, and this was achieved in expense of the transcription of the 3rd chromosomal homolog in males. Thus, apart from X-chromosomal and autosomal dosage compensation, the existence of an X-autosomal dosage compensation in Drosophila is evident in the present work.