X Chromosome Inactivation during Drosophila Spermatogenesis

Abstract

Genes with male- and testis-enriched expression are under-represented on the Drosophila melanogaster X chromosome. There is also an excess of retrotransposed genes, many of which are expressed in testis, that have “escaped” the X chromosome and moved to the autosomes. It has been proposed that inactivation of the X chromosome during spermatogenesis contributes to these patterns: genes with a beneficial function late in spermatogenesis should be selectively favored to be autosomal in order to avoid inactivation. However, conclusive evidence for X inactivation in the male germline has been lacking. To test for such inactivation, we used a transgenic construct in which expression of a lacZ reporter gene was driven by the promoter sequence of the autosomal, testis-specific ocnus gene. Autosomal insertions of this transgene showed the expected pattern of male- and testis-specific expression. X-linked insertions, in contrast, showed only very low levels of reporter gene expression. Thus, we find that X linkage inhibits the activity of a testis-specific promoter. We obtained the same result using a vector in which the transgene was flanked by chromosomal insulator sequences. These results are consistent with global inactivation of the X chromosome in the male germline and support a selective explanation for X chromosome avoidance of genes with beneficial effects late in spermatogenesis.

‘Escaping’ the X chromosome leads to increased gene expression in the male germline of Drosophila melanogaster

“…In combination with previous experiments that showed a reduction in testis expression when an autosomal promoter was moved to the X chromosome (Hense et al, 2007;Kemkemer et al, 2011), our results demonstrate that the X chromosome presents an unfavourable environment with respect to expression in the male germline. The three X-linked promoters used in the current study do not share sequence homology with each other or with other known testis-specific regulatory elements, which suggests that either they do not have a simple, shared regulatory mechanism or that any common regulatory sequences have diverged so extensively that they cannot be detected by a homology search.…”

“…The offsprings of this cross were screened for red eye colour (imparted by the wild-type w þ gene of the vector), which was the diagnostic for stable germline transformants. Additional mobilisations of transgenes to and from the X chromosome were carried out through genetic crosses with the D2-3 transposing-containing stock as described previously (Hense et al, 2007).…”

“…More recent studies have provided empirical support for MSCI in a variety of species, including mammals (Richler et al, 1992;Handel et al, 1994;Turner, 2007), Caenorhabditis elegans (Fong et al, 2002;Kelly et al, 2002) and D. melanogaster (Hense et al, 2007;Vibranovski et al, 2009a). However, there is currently debate regarding the extent of X-linked germline expression silencing in Drosophila and whether it occurs through the same mechanism described as MSCI in other taxa.…”

“…This suggests that at least some level of dosage compensation occurs in mitotic cells of the male germline, and suppression of X-chromosomal gene expression increases at meiosis (Deng et al, 2011, but see Meiklejohn andPresgraves, 2012). Hense et al (2007) showed that autosomal insertions of a transgenic construct containing the promoter of the testis-specific ocnus (ocn) gene fused to a lacZ reporter gene had significantly higher expression than X-linked insertions of the same construct. As the copy number of the reporter gene was the same for both the autosomal and X-linked insertions, these results could not be explained by a lack of dosage compensation and, thus, suggested that another mechanism functions to suppress X-linked gene expression in the male germline.…”

“…As the copy number of the reporter gene was the same for both the autosomal and X-linked insertions, these results could not be explained by a lack of dosage compensation and, thus, suggested that another mechanism functions to suppress X-linked gene expression in the male germline. However, a limitation of the Hense et al (2007) study was that it used only a single promoter sequence that came from an autosomal gene. Thus, it is not known whether the results are relevant to other promoters and, in particular, to promoters of X-linked testis-expressed genes, which presumably have evolved to provide high expression in the male germline.…”