During spermatogenesis, the X chromosome is inactivated in the male germline (sperm cells), thereby silencing, or inactivating, genes residing on the X chromosome. X chromosome silencing is thought to be common among species with XY sex determination and has important implications for genome evolution. For example, genes with increased expressed in the male tend to be underrepresented on the X chromosome, and many testes-specific genes have been retrotransposed, or moved, from the sex to autosomal chromosomes.

However, compelling evidence for X chromosome inactivation in the fruit fly Drosophila has been lacking. Here, Winfried Hense, John Baines, and John Parsch, published in the open-access journal PLoS Biology, used a transgenic technique to test for male germline X inactivation in this important model organism. They randomly inserted a reporter gene whose expression requires a regulatory element for an autosomal testis-specific gene into multiple autosomal and X-chromosomal locations.

They found that autosomal insertions of the reporter gene have significantly higher expression in the male germline than X-linked insertions. This pattern holds for two different transgenes with nearly 50 independent insertions, providing strong evidence for X chromosome inactivation during spermatogenesis. The silencing of X-linked gene expression in the male germline may contribute to the observed paucity of male-expressed genes on the X chromosome and the excess of retrotransposed genes that have moved from the X chromosome to the autosomes to avoid silencing.


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Spermatogenesis X Chromosome Drosophila