Dosage compensation in Drosophila

Abstract

Dosage compensation in Drosophila increases the transcription of genes on the single X chromosome in males to equal that of both X chromosomes in females. Site-specific histone acetylation by the male-specific lethal (MSL) complex is thought to play a fundamental role in the increased transcriptional output of the male X. Nucleation and sequence-independent spreading of the complex to active genes serves as a model for understanding the targeting and function of epigenetic chromatin-modifying complexes. Interestingly, two noncoding RNAs are key for MSL assembly and spreading to active genes along the length of the X chromosome.

Figure 1.

Diagrammatic representation of the results that led H.J. Muller to formulate the hypothesis of dosage compensation. The mutant allele of the X-linked white gene (w^a) is a hypomorph and allows partial eye-pigment synthesis; its presence on the X chromosomes is indicated. The level of pigmentation is directly proportional to the dosage of the w^a allele within each sex; yet, males with one dose and females with two doses have comparable amounts of pigment because of dosage compensation.

Figure 2.

Diagram of the control of sex determination and dosage compensation. If the X/A ratio is equal to 1, a regulatory cascade leads to female sexual development. In females, the presence of the Sxl gene product prevents the translation of the msl2 message and the assembly of the MSL complex. If the X/A ratio is only 0.5, absence of the cascade leads by default to male sexual development and to the formation of the MSL complex.

Figure 3.

The various components of the MSL complex. Known or proposed functions of MSL components include the acetylation of histone H4K16 by MOF and the ubiquitination of H2BK34 by MSL2; MLE has ATPase and RNA/DNA helicase activity, and JIL-1 phosphorylates histone H3. The male-specific complex promotes enrichment of the general factors JIL-1 and topoisomerase II to the male X chromosome.

Figure 4.

Evolutionarily conserved regions of roX RNAs. (A) Gene structure of roX1 and roX2 RNA showing location of stem-loop region (SL) and GUUNUACG roX boxes. (B) Stem-loop structures of the RNAs. (Modified from Maenner et al. 2013, © Elsevier.)

Figure 5.

Localization of the MSL complex. (A) High-resolution ChIP-chip analysis of MSL3 binding and H3K36me3 along 180-kb sections of chromosomes X and 2L in males, illustrated on a logarithmic scale. MSL3 was tagged (MSL-TAP) to purify it using the tandem affinity purification (TAP) technique during ChIP analysis. Concomitant H3K36me3 ChIP-chip analysis shows that the MSL complex colocalizes with H3K36me3 on the middle and 3′ ends of transcribed genes in SL2 cells. Genes on the top line represent those expressed from left to right and those shown below are genes expressed from right to left. Rectangles represent exons, connected by lines that represent introns. Red genes are expressed, whereas black genes are not expressed. (B) Comparison of gene expression state and MSL binding. Genes were divided into quantiles by increasing Affymetrix expression values and graphed to show the percent of genes in each quantile that were clearly bound by MSL complex in ChIP-chip analysis. (C–F) Maximal autosomal spreading is achieved when a roX transgene is the only source of roX RNA in the cell. (C) Chromosomes from a male with a wild-type (WT) X; the presence of an autosomal roX transgene is indicated by the arrow, showing a narrow MSL band (red). X, X chromosome; A, autosome. (D) A male with only one active X-linked roX gene. The MSL complex spreads slightly more from the autosomal roX transgene than in wild type, but binding is reduced on the X chromosome. (E,F) Extensive spreading of the MSL complex in two roX transgenic male lines, in which the X chromosome has both roX genes deleted. (A, Adapted, with permission, from Alekseyenko et al. 2006, © Cold Spring Harbor Laboratory Press; and, with permission, from Larschan et al. 2007, © Elsevier; B, adapted, with permission, from Alekseyenko et al. 2006; C–F, adapted from Park et al. 2002, © AAAS.)

Figure 6.

Targeting of the MSL complex. (A) The MSL complex is found associated with numerous sites along the X chromosome in males (upper panel); a mutant or incomplete complex containing at least MSL1 + MSL2 is found at fewer sites called CESs or HASs. (B) A GA-rich motif is a common feature in MSL CESs. The motif logo is shown with two examples of CESs presented below. This motif occurs once in CES11D1 and three times in CES5C2. The GA-rich core is highlighted in red. (A, Modified from Gu et al. 2000; B, reprinted, with permission, from Alekseyenko et al. 2008, © Elsevier.)

Figure 7.

Correlation of H4K16 acetylation and MSL complex binding on the male X chromosomes. The distribution of H4K16ac on the male X chromosome is broader than MSL complex; active genes that lack stable MSL binding are nonetheless associated with H4K16ac. See Figure 5 for explanation of gene representation. (Adapted from Gelbart et al. 2009.)

Figure 8.

The MSL complex targets activated genes. (A) A construct containing a promoter under the control of the trans-activator GAL4 has been inserted at a site on the X indicated by the yellow arrowhead. This region is normally devoid of the MSL complex in larval salivary gland chromosomes. (B) When GAL4 is introduced, it binds to the construct (red), activates it, and recruits the MSL complex (blue). (Adapted, with permission, from Sass et al. 2003, © National Academy of Sciences.)

Figure 9.

Model for the targeting of the MSL complex to the X chromosome. The dosage compensation complex in Drosophila is proposed to implement at least three targeting principles: (i) interaction with the sites of noncoding roX RNA synthesis, (ii) chromatin context–dependent binding to degenerate DNA sequences of varying affinities, and (iii) DNA sequence–independent movement from initiation sites to chromatin marks signaling gene expression (reviewed in Gelbart and Kuroda 2009). This movement has been characterized as “spreading” based on its apparent restriction in cis to the chromosome of origin, but the underlying molecular mechanism remains to be understood.

Figure 10.

Male-specific conformation of the dosage-compensated X chromosome. A pair of high-affinity chromosomal sites (roX2 and usp) were visualized by two-color FISH (fluorescence in situ hybridization) in female or male embryos. DNA was stained with DAPI (blue) and the X-chromosome territory (magenta) was painted with an antibody against MSL2 in male nuclei (there is no MSL2 in female nuclei). A merge of the channels reveals the proximity of the HASs and their residence relative to the MSL2 territory in male nuclei, clearly summarized in the cartoon on the right. The schematic diagram showing part of the X chromosome below indicates the distances separating the different HASs. (Modified, with permission, from Grimaud and Becker 2009, © Cold Spring Harbor Laboratory Press.)