Fine-scale analysis of X chromosome inactivation in the male germ line of Drosophila melanogaster.

Inactivation of the X chromosome in the male germ line has been suggested to contribute to the excess of gene movement off the X chromosome and the paucity of X-linked male-biased genes that have been observed in Drosophila species. Recent experimental work has demonstrated the transcriptional inactivation of the X chromosome during spermatogenesis, but it is not known if some regions of the X escape inactivation. To test this, we analyzed the expression of 112 precisely-mapped, testis-specific reporter gene insertions along the X chromosome. All of the reporter gene insertions showed low levels of expression that were significantly less than those of autosomal insertions, suggesting that the X chromosome is globally inactivated in the male germ line. There was no evidence for regions of the X chromosome escaping inactivation, including cytological region 19, which appears to be a hot spot for newly evolved, testis-expressed genes.