Hyperactivity of X-chromosome in male Drosophila

The phenomenon of dosage compensation in Drosophila has been shown to be due to hyperactivity .of one X-chromosome in male Drosophila, rather than due to the inactivation of X-chromosome in female. Dr. A.S. Mukherjee of Calcutta University, while working in Beerman's laboratory in Germany, demonstrated that hyperactivity of X-chromosome in maies is responsible for dosage compensation in Drosophila. This was later confirmed by several other workers (see review by Lucchesi, 1973). It was observed that the X-chromosome of male Drosophila in salivary glands was twice as wide as asynapsed autosomes. Through autoradiography also, it could be shown that mRNA synthesis and its translation leads to 100% increase in the activity of a single X-chromosome in male Drosophila. In mosaic individuals with XX and XO cells, it could be shown that X-chromosome in XO cells was always hyperactive. This suggested that this hyperactivity of X does not depend on sex physiology, but on X : A ratio of a cell in the same manner as the sex in Drosophila is determined by X : A ratio. A set of sex linked and autosomal regulatory genes are also involved in this process (see earlier in this section).

A study of certain traits in male and female also shows no difference in male and female flies. While in mice variegation was observed due to random inactivation of X-chromosome in female, no such variegation was observed in female Drosophila. On the other hand, mutant and wild type flies showed same intensity of eye colour in male and female flies. Similarly, enzyme levels for several enzymes including 6PGD (6 phospho-gluconate dehydrogenase), G6PD (glucose-6 phosphate dehydrogenase), 'trytophan pyrrolase' and 'fumarase', were found to be similar in female and male flies. It was also shown that a female fly carrying deficiency for a gene on one of the two X-chromosomes had a lower gene activity than the male fly, although their gene contents were similar. Further, in heterozygous individuals the level of gene activity can be found to be intermediate suggesting that in female Drosophila, inactivation of X-chromosome as suggested in mammals, does not take place.

Another observation in Drosophila was the early DNA replication in X-chromosome of male flies. It is possible that this early replication is a predisposed condition for hyperactivity. This was further confirmed by the observation that the X-chromosome in male has a higher affinity for non-histone chromosomal proteins suggesting that this X-chromosome has higher template activity (non-histone proteins are involved in regulation of gene activity; see Regulation of Gene Expression 3. A Variety of Mechanisms in Eukaryotes).