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

The temporal order of replication of the X chromosome(s) in mitotically dividing male and 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 autosome set in the nucleus. Thus, unlike the well-known early completion of 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 that transcriptional hyperactivity of the single X chromosome required for dosage compensation in somatic cells of male Drosophila is not dependent upon its early replication.