Hyperactivity and faster replicating property of the two arms of the male X of Drosophila pseudoobscura.

The two arms of the X chromosome of Drosophila pseudoobscura have different phylogenetic origin, the XL being homologous to the X and the XR homologous to the 3L of D. melanogaster. The replicative and transcriptive activities of the two arms have been examined in order to understand how such phylogenetically different components of the X contribute toward the chromosomal basis of dosage compensation. The 3H-uridine labelled autoradiograms of the polytene chromosomes of larval salivary glands reveal that the intensity of labelling in the XL and XR of the male is not significantly different from that in the two arms of the female, respectively. The number of grains on the two arms plotted against the grain number on an autosome follows a linear regression, and neither slope in the male is significantly different from its counterpart in the female. The 3H-thymidine autoradiograms show that in all phases of replication, viz. initial, middle and terminal, both arms of the X chromosome in the male are advanced by one step in the cycle. Results, therefore, suggest that both arms of the X of D. pseudoobscura, are hyperactive and faster replicating in the male. Such a situation might arise due to a primary signal coming from autosomally located regulators controlling the super-operon structure of the X chromosomal genes.