Chromosomal basis of dosage compensation in Drosophila: I. Cellular autonomy of hyperactivity of the male X-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 (3L) of larval salivary glands was measured and the rate of RNA synthesis by them was studied upon the use of [³H]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.