X hyperactivation
Open-knowledge reference entry
X hyperactivation refers to the process in Drosophila by which genes on the X chromosome in male flies become twice as active as genes on the X chromosome in female flies.
In Drosophila, there is a stark difference between the X and Y chromosome as most genetic material has been lost on the Y chromosome. Due to this, Drosophila relies on the genetic material of the X chromosome. Because male flies have a single X chromosome and female flies have two X chromosomes, the higher level of activation in males ensures that X chromosome genes are overall expressed at the same level as females. X hyperactivation is one mechanism of dosage compensation, where organisms that use genetic sex determination systems balance the sex chromosome gene dosage between males and females. X hyperactivation is regulated by the alternative splicing of a gene called sex-lethal. The gene was named sex-lethal due to its mutant phenotype which has little to no effect on males but results in the death of females due to X hyperactivation of the two X chromosomes.
In female Drosophila, the sex-lethal protein causes the female-specific splicing of the sex-lethal gene to produce more of the sex-lethal protein. This produces a positive feedback loop. In male Drosophila, there isn’t enough sex-lethal proteins to activate the female-specific splicing of the sex-lethal gene, and it goes through the "default" splicing.
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