Global regulation of X chromosomal genes by the MSL complex in Drosophila melanogaster

Abstract

A long-standing model postulates that X-chromosome dosage compensation in Drosophila occurs by twofold up-regulation of the single male X, but previous data cannot exclude an alternative model, in which male autosomes are down-regulated to balance gene expression. To distinguish between the two models, we used RNA interference to deplete Male-Specific Lethal (MSL) complexes from male-like tissue culture cells. We found that expression of many genes from the X chromosome decreased, while expression from the autosomes was largely unchanged. We conclude that the primary role of the MSL complex is to up-regulate the male X chromosome.Supplemental material is available at http://www.genesdev.org.

roXRNAs Are Required for Increased Expression of X-Linked Genes inDrosophila melanogasterMales

“…Previous studies examining the expression of a limited number of genes in msl mutant larvae produced complex, and sometimes conflicting, data (Bhadra et al 1999(Bhadra et al , 2000(Bhadra et al , 2005Chiang and Kurnit 2003). Two recent studies report a global decrease in X-linked gene expression in Drosophila tissue culture cells following RNA interference (RNAi)-mediated msl2 knockdown (Hamada et al 2005;Straub et al 2005b). We now report that the level 1 of X-linked gene expression is also reduced in roX1 À roX2 À male larvae.…”

“…After observing the nonuniform H4Ac16 redistribution in roX1 À roX2 À males, we decided to normalize microarrays to the median autosomal signals (see materials and methods). This strategy was also based on recent studies documenting reduced X-linked gene expression, rather than increased autosomal expression, following RNAi knockdown of MSL2 in tissue culture cells (Hamada et al 2005;Straub et al 2005b). After normalization and filtering for absent genes, roughly equivalent numbers of X-linked genes were detected in all samples (Table 1).…”

“…These same genes are reduced by $30-40% in our analysis. A global comparison of gene expression between S2 cells lacking MSL2 (Hamada et al 2005) and roX1 À roX2 À male larvae (this report) was also done. About twice as many genes are detected in male larvae as in S2 cells, presumably because larvae have differentiated tissues expressing genes that are silent in tissue culture cells.…”

“…(A) Log 2 ratio of change in expression for autosomal genes in response to loss of both roX genes (x-axis; this study) plotted against log 2 ratio of change in response to MSL2 depletion in S2 cells (y-axis; Hamada et al 2005). (B) Log 2 ratio of change in expression of X-linked genes in response to loss of both roX genes plotted against log 2 ratio of change in S2 cells in response to MSL2 depletion.…”

“…While there is general agreement that this complex is responsible for dosage compensation, disagreement about how compensation is accomplished remains (Birchler et al 2003;Straub et al 2005a). Two recent studies provide strong evidence that binding of the MSL complex to the X chromosome of Drosophila males increases transcription from almost all X-linked genes (Hamada et al 2005;Straub et al 2005b). The MSL complex is composed of five proteins, encoded by the genes maleless (mle), malespecific lethal 1, 2, and 3 (msl1, -2, and -3), and males absent on first (mof ) (Fukunaga et al 1975;Belote and Lucchesi 1980;Uchida et al 1981;Hilfiker et al 1997).…”