Dosage Compensation Mechanisms: Evolution Dosage Compensation Mechanisms: Evolution Ignacio Marı´n, Universidad de Valencia, Valencia, Spain Advanced article Degeneration of sex-limited chromosomes generates the need for dosage compensation Article contents mechanisms able to equalize in both sexes the amount of products derived from sex-linked genes. Evolution of these mechanisms is understood in some detail in the fly Drosophila Dosage Compensation Mechanisms: melanogaster,thenematodeCaenorhabditis elegans and in eutherian mammals. An Evolutionary Perspective Human Dosage Compensation in an Evolutionary Framework Relationships among Dosage Compensation Dosage Compensation Mechanisms: Systems: Common Origin or Convergence? An Evolutionary Perspective doi: 10.1038/npg.els.0006128 In many species, sex chromosomes that originated as expressed at very different levels, in different times and an identical, homologous pair currently show mor- places. However, all available data suggest that general phological differentiation in one of the sexes. An dosage compensation mechanisms are the rule, not the individual of the homogametic sex has two identical exception. We have molecular information for three sex chromosomes, whereas an individual of the dosage compensation systems, those of the fly heterogametic sex has two distinct chromosomes. Drosophila melanogaster, the nematode Caenorhabditis The chromosome restricted to the heterogametic sex elegans and eutherian mammalian species. In these often has a low number of genes. Loss of gene function three cases, the key feature is the existence of a mecha- is caused by the combined effect of chromosome nism able to modify transcriptional levels by modu- restriction to one sex (often due to the emergence of lating chromatin structure along whole chromosomes. a dominant sex-determining gene) and inhibition of A consideration of the mechanisms found in recombination with its homolog. Under these circum- protostome species is relevant here. In D. melanogaster, stances, mutations cannot be eliminated by recombi- females are the homogametic sex (XX), whereas males nation. They therefore accumulate, leading to the are heterogametic (XY). Dosage compensation occurs progressive loss of genetic function in a process via increased expression of X-linked genes in males. It known as chromosome degeneration (reviewed in is achieved by the action of a male-specific ribo- Charlesworth, 1996). (See Sex Chromosomes.) nucleoproteic complex, called a compensasome, that is When chromosome degeneration starts, a mechan- formed by at least five proteins and two noncoding ism known as dosage compensation evolves to equalize RNAs. Compensasomes bind in hundreds of positions the levels of sex chromosome-derived products in both along the male X chromosome. This binding correlates sexes. The discoverer of dosage compensation, H. J. with hypertranscription of X-linked genes. A known Muller, reasoned more than 50 years ago that once the biochemical function of the compensasome is its genes of a chromosome start accumulating mutations, contribution to the enrichment along the male X a progressive need arises to compensate for the loss chromosome of an acetylated isoform of histone H4 of their products. In particular, lowering by half the (acetylated at lysine 16). One of the proteins of the products of genes required in similar levels in both sexes compensasome is a histone acetyltransferase. It is must lead to dysfunctions in the heterogametic sex. still unknown how compensasomes detect their bind- Thus, a dosage compensation mechanism is strongly ing targets on the X chromosome, although there is favored by natural selection. (See Muller, Herman evidence of ‘entry sites’, sequences that recruit com- Joseph.) pensasomes and from which the complexes would There are two ways of generating compensatory spread in cis. mechanisms. As the need for compensation arises in In C. elegans (hermaphrodites XX, males X0), gene parallel to mutation accumulation, and thus in prin- expression in both hermaphrodite X chromosomes is ciple gene-by-gene, independent solutions for each decreased by half. This is the result of the binding of gene could emerge. The alternative is the emergence of a complex of proteins unrelated to those found in a general mechanism able to regulate a twofold the Drosophila compensasome. Which sequences are difference in the expression of many unrelated genes. detected on the X chromosomes for the complex to This option appears unlikely. Genes in a degenerating bind is not yet understood. As in Drosophila, chroma- chromosome are functionally diverse. They are often tin modification is thought to be critical. Two of the ENCYCLOPEDIA OF LIFE SCIENCES & 2005, John Wiley & Sons, Ltd. www.els.net 1 Dosage Compensation Mechanisms: Evolution proteins of the C. elegans dosage compensation inactivation. A large noncoding RNA transcribed from complex belong to a family of proteins involved in the Xist locus accumulates and ‘coats’ the future chromatin condensation in other processes, including inactive chromosome. The gene Tsix, involved in mitosis. However, how condensation is biochemically regulation of Xist function, is transcribed into another achieved is still unknown. noncoding RNA from the strand opposite Xist.AsTsix Comparison of these two systems reveals some overlaps with Xist, its RNA is antisense of the features that are relevant for understanding the origin Xist RNA (hence the peculiar name ‘Tsix’). The and evolution of the mammalian system: (1) compen- most current model suggests that Tsix expression sation is established by global modification of the antagonizes Xist expression, protecting one of the degree of compaction of sex chromosomes; (2) it requ- chromosomes from inactivation. Recently, the first ires the binding of trans-acting factors to sequences on trans-acting regulator has been found. The transcrip- the X chromosomes and (3) the trans-acting factors are tion factor CTCF binds Tsix. It has been suggested different in flies and nematodes, suggesting that these that CTCF binding occurs on only one of the two X mechanisms evolved independently. chromosomes, owing to methylation differences between Tsix alleles. Once CTCF binds and activates Tsix on an X chromosome, Xist function is inhibited, Human Dosage Compensation in allowing that chromosome to remain active (Chao et al., 2002). (See Noncoding RNAs: A Regulatory Role?; an Evolutionary Framework Nonprotein-coding Genes; X-chromosome Inactivation; X-chromosome Inactivation and Disease.) In most eutherian mammals, including humans, However, several lines of evidence suggest that females are the homogametic sex (XX) and males are models including only Xist and Tsix are insufficient. heterogametic (XY). The evolutionary history of For instance, the choice between X chromosomes for human sex chromosomes is complex. They originated inactivation is known to be influenced by the poorly as a pair of homologous autosomes that can be called understood X-controlling element (Xce), which is proto-X and proto-Y. At some moment after the split located nearby but does not overlap with Xist or Tsix. of the lineages (about 320 million years ago), the How Xist RNA spreads along the X chromosome is proto-Y chromosome became restricted to the male also not fully understood. In particular, what kind of sex, probably by the acquisition of a dominant male- cis-acting sequences contribute to this spreading has determining function. There is evidence that, 80–130 not been determined completely. An involvement of million years ago, regions of autosomal origin were LINE-1 repetitive elements has been suggested. These added to both the proto-X and proto-Y chromosomes. elements are enriched in the X chromosome. More- Several chromosomal inversions also occurred, lead- over, some genes are able to escape inactivation and, ing to different gene arrangements in the proto-X and in humans, are concentrated in the regions added most proto-Y, and progressively inhibiting recombination recently to the sex chromosomes. It has been suggested between them. The combination of sex-specific restric- that low concentrations of LINE-1 might define which tion and lack of recombination led to chromosome regions escape inactivation (Bailey et al., 2000). (See degeneration of the proto-Y. Mutations that arose on Long Interspersed Nuclear Elements (LINEs).) the proto-Y could not be eliminated by recombination Once Xist RNA coats an X chromosome, it is in the heterogametic sex. The proto-Y chromosome assumed that it recruits proteins, contributing to the progressively lost most of its genes and became the Y dramatic change in chromatin structure that follows. chromosome of today (Lahn and Page, 1999). (See Soon after Xist spreading, histones H3 and H4 become Chromosome Rearrangement Patterns in Mammalian hypoacetylated, the unusual histone macroH2A1 Evolution; Chromosome X; Chromosome Y; Mammalian accumulates, and gene promoters become hyper- Sex Chromosome Evolution.) methylated. Methylation of histone H3 in its lysine 9 Dosage compensation in embryonic and adult residue, characteristic of heterochromatin in many tissues of eutherians is achieved by random inactiva- eucaryotic organisms, also occurs soon after Xist tion of one of the female X chromosomes. As coating, preceding transcriptional inactivation of happened in the protostome species discussed above, X-linked genes. This may be the first modification to this occurs by changes in chromosome condensation. appear on the future inactive