The hyperactive X-chromosome is not early replicating in mitotically active somatic cells of Drosophila nasuta males

Abstract

The temporal order of repluication of the X chromosome(s) in mitotically dividing male und female cells in early embryos and in brain ganglia of Drosophila nasuta larvae was examined uSing [³H] thymidine pulse labelling and autoradiography. Both the X chromosomes in female cells and the single X chromosome in male cells replicated in complete synchrony with the autosume set in the nucleus. Thus, unlike the well-known early completion or replication by the hemizygous X chromosome in polytene nuclei in the salivary glands of male Drosophila larvae, the single X chromosome in mitotically dividing cells does not replicate earlier than the autosomes. We conclude transcriptional hyperactivity of the single X chromosome required for dosage compensation in somatic cells of male Drosophila is not dependent upon its early replication.