X chromosomes show peculiar features in both gene expression [1
] and gene evolution [2
]. One of the most striking consequences of X chromosome hemizygosity in males, is dosage compensation, a process which brings X chromosome and autosome expression into balance [3
]. Dosage compensation was probably acquired gradually in the course of sex chromosome evolution, as sex chromosomes are thought to arise by divergence of an ancestral autosome pair [6
]. Gene loss from the Y chromosome creates an increasingly aneuploid condition in males and is thought to be the driving force in the evolution of global X-chromosome dosage compensation. In the absence of dosage compensation genomic imbalance results in male lethality.
It has long been known that selective pressures applied to just one of the sexes can effect change in the other [7
]. For example, the coloration of certain birds or the nipples of mammals are advantageous to one of the sexes and are likely to be present as an evolutionary side-effect in the other. X chromosome dosage compensation might also show evidence of this type of sexual selection. X chromosome dosage compensation requires both cis
changes resulting from selection of the compensation system in males will also be present in females, and might alter the character of the X chromosome in females as a secondary consequence [8
]. Indeed, we have previously noted a slight over-expression of both male and female X chromosomes relative to autosomes [4
], which suggests that the X chromosome is inherently more active than autosomes. We have therefore examined the structure of X chromatin in females in detail.
Expression patterns and especially X chromosome dosage compensation are mediated by chromatin modification [1
]. Histones are nucleosome subunits required for packing DNA into the confines of the nucleus. It has long been know that chromatin structure changes are associated with transcription [12
]. For example, when chromatin is physically sheared to small fragments by sonication or enzyme digestion, shearing-bias is associated with different chromatin structures across the genome [13
]. Histones are also modified on N-terminal tail residues to generate an expanding repertoire of histone modifications that are important modulators of transcription [16
]. It has become increasingly clear that specific types of modification are associated with particular transcriptional outcomes. For example, acetylation events are broadly associated with transcriptional activation, while methylation events can have either activating or repressing roles.
One of the best studied histone modifications is the acetylation of Histone 4 on Lysine 16 (H4K16ac). In organisms from yeast to humans, H4K16ac is broadly associated with active genes, and the Histone Acetyl Transferase (HAT) that writes the modification is required for viability [18
]. In Drosophila
, H4K16ac is highly enriched on the X chromosomes of males [22
], and the responsible HAT, Males Absent on First (Mof), is required for male viability [21
]. While Mof is associated with some genes in both males and females [24
], Mof is greatly enriched on the male X chromosome due to targeting by the male-specific-lethal (MSL) complex. MSL is composed of proteins (Mle, Msl1, Msl2, Msl3, and Mof) and two non-coding RNAs encoded on the X (RoX1 and RoX2) [1
]. It is thought that the greatly increased H4K16ac levels act to increase X chromosome expression in males, although it is also possible that X chromosome enrichment depletes autosomes of H4K16ac [1
]. In either model, X chromosome and autosome expression are equilibrated to restore transcription balance.
Another chromatin modifying enzyme, Jil1, is also enriched on the X chromosome of males [26
]. This kinase mediates phosphorylation of Histone 3 at serine 10 (H3S10ph). Jil1 is required for full dosage compensation and associates with the MSL complex [27
]. H3S10ph is implicated in both chromosome condensation during mitosis and transcriptional activation during interphase, suggesting that Jil1 has more general roles in addition to dosage compensation. Another mark associated with active transcription, dimethylation of histone H3 at lysine 4 (H3K4me2) [17
] is general, and thus likely to be MSL complex independent.
We have performed chromatin-shearing experiments showing that X chromatin differs from autosomal chromatin in both males and females. Additionally, the histone marks associated with X chromosome dosage compensation in males are modestly enriched on female X chromosomes. These data indicate that X chromatin is distinct even in the absence of dosage compensation. We suggest that the pattern in females is a tolerated neutral side-effect of the evolution of X chromosome dosage compensation in males.