Chromosomal basis of dosage compensation in<i>Drosophila</i>: I. Cellular autonomy of hyperactivity of the male<i>X</i>-chromosome in salivary glands and sex differentiation

Abstract

Morphology and the rate of RNA synthesis of the X -chromosome in XX/XO mosaic larval salivary glands of Drosophila melanogaster have been examined. For this purpose the unstable ring- X was utilized to produce XX and XO nuclei in the same pair of glands. The width of the X -chromosome and the left arm of the 3rd chromosome (3 L ) of larval salivary glands was measured and the rate of RNA synthesis by them was studied upon the use of [ 3 H]uridine autoradiography in such XX (female) and XO (male) nuclei developing in a female background (i.e. otherwise genotypically XX ). In such mosaic glands the width of the single X -chromosome of male nuclei is nearly as great as that of the paired two X 's of female nuclei, as is also the case in normal male ( X Y ) and female ( XX ). The single X of male nuclei synthesizes RNA at a rate equal to that of the paired two X 's of female nuclei and nearly twice that of an unpaired X of XX nuclei. Neither the developmental physiology of the sex nor the proportion of XO nuclei in a pair of mosaic salivary glands of an XX larva has any influence on these two characteristics of the male X -chromosome. It is suggested that dosage compensation in Drosophila is achieved chiefly, if not fully, by a hyperactivity of the male X , in contrast to the single X inactivation in female mammals, that this hyperactivity of the male X is expressed visibly in the morphology and metabolic activity of the X -chromosome in the larval salivary glands of the male, and that this hyperactivity and therefore dosage compensation in Drosophila in general is not dependent on sex-differentiation, but is a function of the doses of the X -chromosome itself.