X Chromosome Inactivation During Drosophila Spermatogenesis

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.



Citation: Hense W, Baines JF, Parsch J (2007) X chromosome inactivation
during Drosophila spermatogenesis. PLoS Biol 5(10): e273.
doi:10.1371/journal.pbio.0050273

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