Dosage Compensation in Drosophila-a Model for the Coordinate Regulation of Transcription

Abstract

The sex chromosomes have special significance in the history of genetics. The chromosomal basis of inheritance was firmly established when Calvin Bridges demonstrated that exceptions to Mendel's laws of segregation were accompanied at the cytological level by exceptional sex chromosome segregation. The morphological differences between X and Y exploited in Bridges' experiments arose as a consequence of the evolution of the sex chromosomes. Originally a homologous chromosome pair, the degeneration of the Y chromosome has been accompanied by a requirement for increased expression of the single X chromosome in males. Drosophila has been a model for the study of this dosage compensation and has brought key strengths, including classical genetics, the exceptional cytology of polytene chromosomes, and more recently, comprehensive genomics. The impact of these studies goes beyond sex chromosome regulation, providing valuable insights into mechanisms for the establishment and maintenance of chromatin domains, and for the coordinate regulation of transcription.

Keywords: Drosophila; FlyBook; X chromosome; dosage dependence; genetic screens; male-specific lethal.

Figure 1

Sex-specific regulation of dosage compensation in Drosophila by the X chromosome to autosome ratio. In diploid females (left), the ratio is 1.0 (XX:2A), which represents a basal or “balanced” state of gene expression. The ratio of 1.0 triggers expression of SXL, which positively regulates female-specific splicing and transcriptional regulators (TRA and DSX^F) while repressing the translation of MSL2. Lack of MSL2 prevents inappropriate MSL-complex formation in females. In diploid males (right), the X:A ratio is 0.5 (XY:2A) leading to selective pressure to upregulate X-linked genes. SXL is not expressed, therefore MSL2 is translated, the full MSL complex forms, associates with X-linked genes, and increases their transcription. The lack of SXL also results in male-specific differentiation through expression of DSX^M (see Cline and Meyer 1996).

Figure 2

The w^a mutant is a partial loss-of-function allele that allows the deposition of some eye pigments in the eye. The greater the number of w^a alleles in the genotype of either females (top row of karyotypes) or males (bottom row of karyotypes) the greater the amount of pigmentation. Surprisingly, females with two doses and males with a single dose of the allele have the same eye color. From Muller 1948; reprinted with permission from The Harvey Society.

Figure 3

Overview of selected Drosophila karyotypes. (A) Diagram of the six chromosomal elements present in various configurations in different species of the genus Drosophila. (B) Male of D. melanogaster. (C) Male of D. pseudoobscura. One of the autosomal elements is fused to the ancestral X, yielding a metacentric X chromosome. (D) Female of D. pseudoobscura or D. miranda. (E) Male of D. miranda. A different autosomal element is fused to the Y chromosome and is in the process of becoming inactivated and heterochromatic. From Lucchesi 1978 reprinted with permission from John Wiley and Sons (New York).

Figure 4

MSL proteins localize to the male X chromosome. MSL3 (red) is present in a reproducible interband pattern on the larval polytene male X chromosome, and not on the autosomes (blue). From Alekseyenko et al. 2006; reprinted with permission from Cold Spring Harbor Laboratory Press (Cold Spring Harbor, NY).

Figure 5

MSL proteins spread from roX transgenes inserted on autosomes. (Top) Diagram of a roX1 roX2 double-mutant X chromosome (X) and an autosome (A) with a 5-kb wild-type roX⁺ transgene insertion. (Bottom) Anti-MSL1 staining of male polytene chromosomes carrying the roX⁺ transgene insertion on an autosome. MSL staining (red) is seen on the roX1 roX2 mutant X chromosome, but also up to 1 Mb in cis to the transgenic insertion (adapted from Park et al. 2002).

Figure 6

Model for MSL targeting of the X chromosome. Initial assembly occurs on the sites of roX RNA synthesis and may nucleate at CESs before spreading in cis to produce the wild-type MSL pattern on the X chromosome. Assembly is limited to CES in the absence of MSL3 protein (adapted from Kelley et al. 1999).

Figure 7

Diagram of the two roX RNAs showing the location of the conserved sequences (roX boxes) and structural features (stem loops). The sequences forming the stem loops are different in the two RNAs except for one copy of the roX box which they share. From Maenner et al. 2013; reprinted with permission from Elsevier (Amsterdam).

Figure 8

MSL complex associates with the bodies of active X-linked genes with a 3′ bias. The metagene profiles show X-linked genes with the greatest MSL occupancy in red (bound genes, n = 700, the vast majority of which are transcribed), compared to unbound genes in green (n = 702, largely not expressed), or to all genes on the 2L autosomal arm in gray. The average bound profile covers the bodies of active genes on the male X, with a 3′ bias. All transcription units were scaled and aligned at their 5′ and 3′ ends. Chr, chromosome. From Alekseyenko et al. 2006; reprinted with permission from Cold Spring Harbor Laboratory Press (Cold Spring Harbor, NY).

Figure 9

Recovery of the CESs and their consensus sequence. Crossing scheme that results in recovery of msl3 mutant male embryos, along with their msl3⁺ sisters. Since only males express MSL2, ChIP of the total population results in selective recovery of MSL2 sites remaining in the absence of MSL3, also known as the CESs. The Multiple EM for Motif Elicitation algorithm revealed a consensus MRE logo based on the 150 sites recovered (adapted from Alekseyenko et al. 2008).

Figure 10

Jumpstart and gain model for dosage compensation. The transcription profile of genes on autosomes (Auto, blue line) compared to dosage compensated genes on X (chrX, red line). Both attract abundant Pol II, producing short transcripts while paused. An increase of mRNA production occurs on X-linked genes due to H4K16ac enriched on gene bodies. The facilitated steps include pause release (jumpstart), and a measureable gain during elongation. From Ferrari et al. 2013b; reprinted with permission from Elsevier (Amsterdam).