In order to determine the level of X-linked gene activity in metamales (XY; 3A) of Drosophila melanogaster, RNA synthesis was monitored as a function of 3H-uridine incorporation in nuclei of isolated larval salivary glands by standard autoradiographic means. These data show that the rate of RNA synthesis by the single X chromosome in metamales is approximately 90% of the rate achieved by the three X chromosomes of triploid females (3X; 3A). As an independent measure of chromosome activity, two X-linked enzymes and two autosomal enzymes were monitored in crude extracts by spectrophotometric assay procedures. The results reveal that X-linked enzyme activity levels in metamales constitute 80% of the levels exhibited by control triploid females or intersexes (XXY; 3A). We conclude that metamales exhibit substantial compensation beyond that which occurs in regular, diploid males. This finding establishes a new level of transcriptional activity possible for the X chromosome of Drosophila melanogaster.
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