Chromosomal basis of dosage compensation inDrosophila: I. Cellular autonomy of hyperactivity of the maleX-chromosome in salivary glands and sex differentiation

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Abstract
Morphology and the rate of RNA synthesis of theX-chromosome inXX/XOmosaic larval salivary glands ofDrosophila melanogasterhave been examined. For this purpose the unstable ring-Xwas utilized to produceXXandXOnuclei in the same pair of glands. The width of theX-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 suchXX(female) andXO(male) nuclei developing in a female background (i.e. otherwise genotypicallyXX). In such mosaic glands the width of the singleX-chromosome of male nuclei is nearly as great as that of the paired twoX's of female nuclei, as is also the case in normal male (X Y) and female (XX). The singleXof male nuclei synthesizes RNA at a rate equal to that of the paired twoX's of female nuclei and nearly twice that of an unpairedXofXXnuclei. Neither the developmental physiology of the sex nor the proportion ofXOnuclei in a pair of mosaic salivary glands of anXXlarva has any influence on these two characteristics of the maleX-chromosome.It is suggested that dosage compensation inDrosophilais achieved chiefly, if not fully, by a hyperactivity of the maleX, in contrast to the singleXinactivation in female mammals, that this hyperactivity of the maleXis expressed visibly in the morphology and metabolic activity of theX-chromosome in the larval salivary glands of the male, and that this hyperactivity and therefore dosage compensation inDrosophilain general is not dependent on sex-differentiation, but is a function of the doses of theX-chromosome itself.