Dosage compensation of X-chromosome activity in interspecific hybrids of Drosophila melanogaster and D. simulans

We have used the unstable ring X-chromosome of D. melanogaster to generate XX/XO mosaics in the hybrid progeny from crosses between D. melanogaster females and D. simulans males. The functional properties of the polytene X-chromosome(s) in salivary glands of such XO/XX mosaic hybrid larvae have been analysed by autoradiography after 3H-uridine or 3H-thymidine labelling of the glands. The simulans X-chromosome in the hybrid XO nuclei displays typical pale staining, enlarged diameter, higher rate of transcription (nearly two times higher than each of the Xs in the XX nuclei in the same gland) and a faster completion of replication as would be the case in the original parental XO or XY nuclei. In the hybrid XX polytene nuclei, the melanogaster as well as the simulans X functions in the same manner as in female cells of the parents. The nucleolar transcription is also equal in the hybrid XX and XO nuclei. Thus it seems that despite the evolutionary diversification between these two species, the regulatory system which brings about the dosage compensation of X-chromosome activity has been conserved.