Showing papers on "Dosage compensation published in 2004"

Extensive gene traffic on the mammalian X chromosome.

Abstract: Mammalian sex chromosomes have undergone profound changes since evolving from ancestral autosomes. By examining retroposed genes in the human and mouse genomes, we demonstrate that, during evolution, the mammalian X chromosome has generated and recruited a disproportionately high number of functional retroposed genes, whereas the autosomes experienced lower gene turnover. Most autosomal copies originating from X-linked genes exhibited testis-biased expression. Such export is incompatible with mutational bias and is likely driven by natural selection to attain male germline function. However, the excess recruitment is consistent with a combination of both natural selection and mutational bias.

The mouse X chromosome is enriched for sex-biased genes not subject to selection by meiotic sex chromosome inactivation.

Abstract: Sex chromosomes are subject to sex-specific selective evolutionary forces. One model predicts that genes with sex-biased expression should be enriched on the X chromosome. In agreement with Rice's hypothesis, spermatogonial genes are over-represented on the X chromosome of mice and sex- and reproduction-related genes are over-represented on the human X chromosome. Male-biased genes are under-represented on the X chromosome in worms and flies, however. Here we show that mouse spermatogenesis genes are relatively under-represented on the X chromosome and female-biased genes are enriched on it. We used Spo11(-/-) mice blocked in spermatogenesis early in meiosis to evaluate the temporal pattern of gene expression in sperm development. Genes expressed before the Spo11 block are enriched on the X chromosome, whereas those expressed later in spermatogenesis are depleted. Inactivation of the X chromosome in male meiosis may be a universal driving force for X-chromosome demasculinization.

Resolution and evolution of the duck-billed platypus karyotype with an X1Y1X2Y2X3Y3X4Y4X5Y5 male sex chromosome constitution

Abstract: The platypus (2n = 52) has a complex karyotype that has been controversial over the last three decades. The presence of unpaired chromosomes and an unknown sex-determining system especially has defied attempts at conventional analysis. This article reports on the preparation of chromosome-specific probes from flow-sorted chromosomes and their application in the identification and classification of all platypus chromosomes. This work reveals that the male karyotype has 21 pairs of chromosomes and 10 unpaired chromosomes (E1-E10), which are linked by short regions of homology to form a multivalent chain in meiosis. The female karyotype differs in that five of these unpaired elements (E1, E3, E5, E7, and E9) are each present in duplicate, whereas the remaining five unpaired elements (E2, E4, E6, E8, and E10) are absent. This finding indicates that sex is determined by the alternate segregation of the chain of 10 during spermatogenesis so that equal numbers of sperm bear either one of the two groups of five elements, i.e., five X and five Y chromosomes. Chromosome painting reveals that these X and Y chromosomes contain pairing (XY shared) and differential (X- or Y-specific) segments. Y differential regions must contain male-determining genes, and X differential regions should be dosage-compensated in the female. Two models for the evolution of the sex-determining system are presented. The resolution of the longstanding debate over the platypus karyotype is an important step toward the understanding of mechanisms of sex determination, dosage compensation, and karyotype evolution.

Developmentally regulated alterations in Polycomb repressive complex 1 proteins on the inactive X chromosome.

Abstract: Polycomb group (PcG) proteins belonging to the polycomb (Pc) repressive complexes 1 and 2 (PRC1 and PRC2) maintain homeotic gene silencing. In Drosophila, PRC2 methylates histone H3 on lysine 27, and this epigenetic mark facilitates recruitment of PRC1. Mouse PRC2 (mPRC2) has been implicated in X inactivation, as mPRC2 proteins transiently accumulate on the inactive X chromosome (Xi) at the onset of X inactivation to methylate histone H3 lysine 27 (H3-K27). In this study, we demonstrate that mPRC1 proteins localize to the Xi, and that different mPRC1 proteins accumulate on the Xi during initiation and maintenance of X inactivation in embryonic cells. The Xi accumulation of mPRC1 proteins requires Xist RNA and is not solely regulated by the presence of H3-K27 methylation, as not all cells that exhibit this epigenetic mark on the Xi show Xi enrichment of mPRC1 proteins. Our results implicate mPRC1 in X inactivation and suggest that the regulated assembly of PcG protein complexes on the Xi contributes to this multistep process.

X-chromosome inactivation patterns are unbalanced and affect the phenotypic outcome in a mouse model of rett syndrome.

Abstract: Rett syndrome (RTT), a neurodevelopmental disorder affecting mostly females, is caused by mutations in the X-linked gene encoding methyl-CpG–binding protein 2 (MeCP2). Although the majority of girls with classic RTT have a random pattern of X-chromosome inactivation (XCI), nonbalanced patterns have been observed in patients carrying mutant MECP2 and, in some cases, account for variability of phenotypic manifestations. We have generated an RTT mouse model that recapitulates all major aspects of the human disease, but we found that females exhibit a high degree of phenotypic variability beyond what is observed in human patients with similar mutations. To evaluate whether XCI influences the phenotypic outcome of Mecp2 mutation in the mouse, we studied the pattern of XCI at the single-cell level in brains of heterozygous females. We found that XCI patterns were unbalanced, favoring expression of the wild-type allele, in most mutant females. It is notable that none of the animals had nonrandom XCI favoring the mutant allele. To explore why the XCI patterns favored expression of the wild-type allele, we studied primary neuronal cultures from Mecp2-mutant mice and found selective survival of neurons in which the wild-type X chromosome was active. Quantitative analysis indicated that fewer phenotypes are observed when a large percentage of neurons have the mutant X chromosome inactivated. The study of neuronal XCI patterns in a large number of female mice carrying a mutant Mecp2 allele highlights the importance of MeCP2 for neuronal viability. These findings also raise the possibility that there are human females who carry mutant MECP2 alleles but are not recognized because their phenotypes are subdued owing to favorable XCI patterns.

Recruitment and spreading of the C. elegans dosage compensation complex along X chromosomes

Abstract: To achieve X-chromosome dosage compensation, organisms must distinguish X chromosomes from autosomes. We identified multiple, cis-acting regions that recruit the Caenorhabditis elegans dosage compensation complex (DCC) through a search for regions of X that bind the complex when detached from X. The DCC normally assembles along the entire X chromosome, but not all detached regions recruit the complex, despite having genes known to be dosage compensated on the native X. Thus, the DCC binds first to recruitment sites, then spreads to neighboring X regions to accomplish chromosome-wide gene repression. From a large chromosomal domain, we defined a 793-base pair fragment that functions in vivo as an X-recognition element to recruit the DCC.

Functional integration of the histone acetyltransferase MOF into the dosage compensation complex.

Abstract: Dosage compensation in flies involves doubling the transcription of genes on the single male X chromosome to match the combined expression level of the two female X chromosomes. Crucial for this activation is the acetylation of histone H4 by the histone acetyltransferase (HAT) MOF. In male cells, MOF resides in a complex (dosage compensation complex, DCC) with MSL proteins and noncoding roX RNA. Previous studies suggested that MOF's localization to the X chromosome was largely RNA-mediated. We now found that contact of the MOF chromo-related domain with roX RNA plays only a minor role in correct targeting to the X chromosome in vivo. Instead, a strong, direct interaction between a conserved MSL1 domain and a zinc finger within MOF's HAT domain is crucial. The functional consequences of this interaction were studied in vitro. Simultaneous contact of MOF with MSL1 and MSL3 led to its recruitment to chromatin, a dramatic stimulation of HAT activity and to improved substrate specificity. Activation of MOF's HAT activity upon integration into the DCC may serve to restrict the critical histone modification to the male X chromosome.

De novo DNA methylation is dispensable for the initiation and propagation of X chromosome inactivation

Abstract: Xist (X-inactive specific transcript) plays a crucial role in X-inactivation. This non-coding RNA becomes upregulated on the X chromosome that is to be inactivated upon differentiation. Previous studies have revealed that although maintenance-type DNA methylation is not essential for X-inactivation to occur, it is required for the stable repression of Xist in differentiated cells. However, it is unknown whether differential de novo methylation at the Xist promoter, which is mediated by Dnmt3a and/or Dnmt3b, is a cause or a consequence of monoallelic expression of Xist. We show that Xist expression is appropriately regulated in the absence of Dnmt3a and Dnmt3b and that a single X chromosome undergoes proper inactivation in mutant females. Our results indicate that a mechanism(s) other than DNA methylation plays a principal role in initiating X-inactivation. We also demonstrate that delayed upregulation of Xist does not induce X-inactivation, consistent with a crucial developmental window for the chromosomal silencing.

Polytene chromosomes: 70 years of genetic research.

Abstract: Polytene chromosomes were described in 1881 and since 1934 they have served as an outstanding model for a variety of genetic experiments. Using the polytene chromosomes, numerous biological phenomena were discovered. First the polytene chromosomes served as a model of the interphase chromosomes in general. In polytene chromosomes, condensed (bands), decondensed (interbands), genetically active (puffs), and silent (pericentric and intercalary heterochromatin as well as regions subject to position effect variegation) regions were found and their features were described in detail. Analysis of the general organization of replication and transcription at the cytological level has become possible using polytene chromosomes. In studies of sequential puff formation it was found for the first time that the steroid hormone (ecdysone) exerts its action through gene activation, and that the process of gene activation upon ecdysone proceeds as a cascade. Namely on the polytene chromosomes a new phenomenon of cellular stress response (heat shock) was discovered. Subsequently chromatin boundaries (insulators) were discovered to flank the heat shock puffs. Major progress in solving the problems of dosage compensation and position effect variegation phenomena was mainly related to studies on polytene chromosomes. This review summarizes the current status of studies of polytene chromosomes and of various phenomena described using this successful model.

Aneuploidy and copy number breakpoints in the genome of lager yeasts mapped by microarray hybridisation.

Abstract: Competitive comparative genome hybridisation (CCGH) to Saccharomyces cerevisiae DNA microarrays and quantitative real-time polymerase chain reaction (qRT-PCR) assays are used to examine the copy number of S. cerevisiae-like genes, at single gene resolution, of two bottom-fermenting lager yeast strains, CMBS-33 and 6701. Using the S. cerevisiae gene order for each chromosome, we observe that the copy number for contiguous groups of S. cerevisiae-like genes is similar in both strains. However, discrete changes in copy number occur at distinct loci, indicating the aneuploid nature of the lager yeast genomes. The majority of loci where copy number changes occur are conserved in both strains. We also identify a large segment of S. cerevisiae DNA on chromosome XVI that fails to hybridise to genomic DNA from both lager strains, suggesting that this region may have diverged significantly or is absent in the lager yeast strains. Furthermore, very low levels of mRNA transcripts are detected from this region of the genome. Interestingly, the increased gene copy number observed elsewhere (e.g. chromosome III) does not correlate specifically with increased gene expression under fermentation conditions, suggesting that dosage compensation may play a role in controlling gene expression in these strains.

Characterization of loss-of-inactive X in Klinefelter syndrome and female-derived cancer cells.

Abstract: The increased risk of several types of cancer in Klinefelter syndrome (47XXY) suggests that the extra X chromosome may be involved in the tumorigenesis associated with this syndrome. Here, we show that cancer cells (PSK-1) derived from a patient with Klinefelter syndrome (47XXY) showing loss of an inactive X chromosome subsequently gained active X chromosomes. We found that this abnormal X chromosome composition in PSK-1 is caused by a loss of an inactive X chromosome followed by multiplication of identical active X chromosomes, not by reactivation of an inactive X chromosome. Furthermore, we extended the characterization of loss-of-inactive X in a series of 22 female-derived cancer cell lines (eight breast cancer cell lines, seven ovarian cancer cell lines, and seven cervical cancer cell lines). The data demonstrate that loss-of-inactive X in the female-derived cancer cells is mainly achieved by loss of an inactive X chromosomes followed by multiplication of an identical active X chromosomes. However, distinctive pathways, including reactivation of an inactive X chromosome, are also involved in the mechanisms for loss-of-inactive X and gain-of-active X in female-derived cancer cells. The biological significance of the loss-of-inactive X and gain-of-active X in the oncogenesis of Klinefelter syndrome and female-derived cancer cells are discussed.

X chromosome sites autonomously recruit the dosage compensation complex in Drosophila males.

Abstract: It has been proposed that dosage compensation in Drosophila males occurs by binding of two core proteins, MSL-1 and MSL-2, to a set of 35-40 X chromosome "entry sites" that serve to nucleate mature complexes, termed compensasomes, which then spread to neighboring sequences to double expression of most X-linked genes. Here we show that any piece of the X chromosome with which compensasomes are associated in wild-type displays a normal pattern of compensasome binding when inserted into an autosome, independently of the presence of an entry site. Furthermore, in chromosomal rearrangements in which a piece of X chromosome is inserted into an autosome, or a piece of autosome is translocated to the X chromosome, we do not observe spreading of compensasomes to regions of autosomes that have been juxtaposed to X chromosomal material. Taken together these results suggest that spreading is not involved in dosage compensation and that nothing distinguishes an entry site from the other X chromosome sites occupied by compensasomes beyond their relative affinities for compensasomes. We propose a new model in which the distribution of compensasomes along the X chromosome is achieved according to the hierarchical affinities of individual binding sites.

Ubiquitinated proteins including uH2A on the human and mouse inactive X chromosome: enrichment in gene rich bands

Abstract: The inactive X chromosome (Xi) forms a heterochromatic structure in the nucleus that is known to have several modifications to specific histones involving acetylation or methylation. Using three different antibodies in four different cell lines, we demonstrate that the Xi in human and mouse cells is highly enriched in ubiquitinated protein(s), much of which is polyubiquitinated. This ubiquitination appears specific for the Xi as it was not observed for centromeres or other regions of heterochromatin. Results using an antibody specific to ubiquitinated H2A provide a clear link between H2A ubiquitination and gene repression, as visualized across an entire inactive chromosome. Interestingly, the ubiquitination of the chromosome persists into mitosis and can be seen in a reproducible banded pattern. This pattern matches that of Xist RNA which forms bands as it detaches from the mitotic X chromosome. Both ubiquitination and Xist RNA appear enriched in gene dense regions and depleted in gene poor bands, but do not correlate with L1 LINE elements which have been suggested as key to X-inactivation. These results provide evidence that ubiquitination along with Xist RNA plays an important role in the formation of facultative heterochromatin during X-inactivation.

Disruption of a new X linked gene highly expressed in brain in a family with two mentally retarded males.

Abstract: Background: Mental retardation (MR) affects 2-3% of the human population and some of these cases are genetically determined. Although several genes responsible for MR have been identified, many cases have still not been explained. Methods: We have identified a pericentric inversion of the X chromosome inv(X)(p22.3;q13.2) segregating in a family where two male carriers have severe MR while female carriers are not affected. Results: The molecular characterisation of this inversion led us to identify two new genes which are disrupted by the breakpoints: KIAA2022 in Xq13.2 and P2RY8 in Xp22.3. These genes were not previously fully characterised in humans. KIAA2022 encodes a protein which lacks significant homology to any other known protein and is highly expressed in the brain. P2RY8 is a member of the purine nucleotide G-protein coupled receptor gene family. It is located in the pseudo-autosomal region of the X chromosome and is not expressed in brain. Conclusions: Because the haploinsufficiency of P2RY8 in carrier mothers does not have a phenotypic consequence, we propose that the severe MR of the affected males in this family is due to the absence of the KIAA2022 gene product. However, screening 20 probands from X linked MR families did not reveal mutations in KIAA2022. Nonetheless, the high expression of this gene in fetal brain and in the adult cerebral cortex could be consistent with a role in brain development and/or cognitive function.

Shared forces of sex chromosome evolution in haploid-mating and diploid-mating organisms: Microbotryum violaceum and other model organisms.

Abstract: It is usually posited that the most important factors contributing to sex chromosome evolution in diploids are the suppression of meiotic recombination and the asymmetry that results from one chromosome (the Y) being permanently heterozygous and the other (the X) being homozygous in half of the individuals involved in mating. To distinguish between the roles of these two factors, it would be valuable to compare sex chromosomes in diploid-mating organisms and organisms where mating compatibility is determined in the haploid stage. In this latter group, no such asymmetry occurs because the sex chromosomes are equally heterozygous. Here we show in the fungus Microbotryum violaceum that the chromosomes carrying the mating-type locus, and thus determining haploid-mating compatibility, are rich in transposable elements, dimorphic in size, and carry unequal densities of functional genes. Through analysis of available complete genomes, we also show that M. violaceum is, remarkably, more similar to humans and mice than to yeast, nematodes, or fruit flies with regard to the differential accumulation of transposable elements in the chromosomes determining mating compatibility vs. the autosomes. We conclude that restricted recombination, rather than asymmetrical sheltering, hemizygosity, or dosage compensation, is sufficient to account for the common sex chromosome characteristics.

Painting of fourth in genus Drosophila suggests autosome-specific gene regulation.

Abstract: Painting of fourth (POF) is a chromosome-specific protein in Drosophila and represents the first example of an autosome-specific protein. POF binds to chromosome 4 in Drosophila melanogaster, initiating at the proximal region, followed by a spreading dependent on chromosome 4-specific sequences or structures. Chromosome-specific gene regulation is known thus far only as a mechanism to equalize the transcriptional activity of the single male X chromosome with that of the two female X chromosomes. In Drosophila, a complex including the male-specific lethal proteins, "paints" the male X chromosome, mediating its hypertranscription, explained to some extent by the acetylation of lysine 16 on histone H4. Here, we show that Pof is essential for viability in both sexes and for female fertility. POF binding to an autosome, the F element, is conserved in genus Drosophila, indicating functional conservation of the autosome specificity. In three of nine studied species, POF binds to the male X chromosome. When bound to the male X, it also colocalizes with the dosage compensation protein male-specific lethal 3, suggesting a relationship to dosage compensation. The chromosome specificity is determined at the species level and not by the amino acid sequence. We argue that POF is involved in a chromosome-specific regulatory function.

Battle of the Xs.

Abstract: Females and males often exhibit conspicuous morphological, physiological and behavioral differences. Similarly, gene expression profiles indicate that a large portion of the genome is sex-differentially deployed, particularly in the germ line. Because males and females are so fundamentally different, each sex is likely to have a different optimal gene expression profile that is never fully achieved in either sex because of antagonistic selection in females versus males. Males are hemizygous for the X chromosome, which means that recessive male-favorable de novo mutations on the X chromosome are subject to immediate selection. In females, a recessive female-favorable mutation on one of two X chromosomes is not available for selection until it becomes frequent enough in the local population to result in homozygous individuals. Given that most mutations are recessive, one would expect that genes or alleles favoring males should accumulate on the X chromosome. Recent microarray work in Drosophila and C. elegans clearly shows the opposite. Why is the X chromosome a highly disfavored location for genes with male-biased expression in these animals? BioEssays 26:543–548, 2004. Published 2004 Wiley Periodicals, Inc.

Association of BRCA1 with the inactive X chromosome and XIST RNA.

Abstract: Breast cancer, early onset 1 (BRCA1) encodes a nuclear protein that participates in breast and ovarian cancer suppression. The molecular basis for the gender and tissue specificity of the BRCA1 cancer syndrome is unknown. Recently, we observed that a fraction of BRCA1 in female cells is localized on the inactive X chromosome (Xi). Chromatin immunoprecipitation (ChIP) experiments have demonstrated that BRCA1 physically associates with Xi-specific transcript (XIST) RNA, a non-coding RNA known to coat Xi and to participate in the initiation of its inactivation during early embryogenesis. Cells lacking wild-type BRCA1 show abnormalities in Xi, including lack of proper XIST RNA localization. Reintroduction of wild-type, but not mutant, BRCA1 can correct this defect in XIST localization in these cells. Depletion of BRCA1 in female diploid cells led to a defect in proper XIST localization on Xi and in the development of normal Xi heterchromatic superstructure. Moreover, depletion of BRCA1 led to an increased likelihood of re-expression of a green fluorescent protein (GFP) reporter gene embedded on Xi. Taken together, these findings are consistent with a model in which BRCA1 function contributes to the maintenance of proper Xi heterochromatin superstructure. Although the data imply a novel gender-specific consequence of BRCA1 loss, the relevance of the BRCA1/Xi function to the tumour suppressor activity of BRCA1 remains unclear and needs to be tested.

Role of de novo DNA methyltransferases in initiation of genomic imprinting and X-chromosome inactivation.

Abstract: plays a key role in regulation of developmental gene expression, maintenance of genomic integrity, genomic imprinting, and X-chromosome inactivation (X-inactivation) in mammals. Methylation of mammalian genomic DNA occurs almost exclusively at the cytosine of CpG dinucleotides. The CpG methylation pattern of the mammalian genome is created and maintained by a combination of de novo DNA methyltransferases, Dnmt3a and Dnmt3b, and a maintenance DNA methyltransferase Dnmt1. Targeted disruption of these DNA methyltransferase genes in mice results in embryonic or early postnatal lethality, indicating that they are essential for normal mammalian development (Li et al. 1992; Okano et al. 1999). Genomic imprinting and X-inactivation are the wellcharacterized, major epigenetic phenomena of mammals that regulate allelic expression of autosomal genes and Xlinked genes, respectively (Lyon 1961; Reik and Walter 2001). Both phenomena are known to be crucial for normal mammalian development. Imprinting is initiated during male and female gametogenesis, marking a subset of autosomal genes (up to a few hundred) in a sex-specific way (paternal and maternal imprinting). The imprinted genes show either paternal-specific or maternal-specific monoallelic expression in the offspring (Reik and Walter 2001). Thus imprinting is dependent on the sex of the parent from which the gene is derived, but not on the sex of the individual that carries the gene. By contrast, X-inactivation is a dosage compensation mechanism found only in females, which equalizes the X-linked gene dosage between males (with one X and one Y chromosome) and females (with two X chromosomes) (Lyon 1961). In the embryo proper (the epiblast lineages), X-inactivation is initiated during early development, leading to random inactivation of either the paternal or the maternal X chromosome. However, in the extraembryonic lineages (trophoblast and primitive endoderm derivatives) of mice, preferential inactivation of the paternal X chromosome occurs (Takagi and Sasaki 1975). Thus X-inactivation can be subject to genomic imprinting (imprinted X-inactivation). Like the imprinting of autosomes, the imprinting of X chromosome is thought to occur in the parental germ line. Previous studies with the mouse embryos and ES cells deficient for Dnmt1 showed that DNA methylation plays an essential role in the maintenance of genomic imprinting and X-inactivation in the embryo proper (Table 1) (Li et al. 1993; Beard et al. 1995; Panning and Jaenisch, 1996; Sado et al. 2000). By contrast, in the trophoblast, the role of DNA methylation seems more relaxed (Table 1) (Caspery et al. 1998; Tanaka et al. 1999; Sado et al. 2000). However, whether DNA methylation is involved in their initiation has not been addressed. If DNA methylation were to play a role in the initiation step, Dnmt3a or Dnmt3b (or both) should be the key players because these are the enzymes that establish new genomic methylation patterns (Okano et al. 1999). We therefore asked whether Dnmt3a and/or Dnmt3b is involved in the initiation of autosomal imprinting and X-inactivation using the cells and embryos deficient for these genes. The Cre-loxP conditional gene knockout system was particularly useful because of the early lethality of conventional Dnmt3a or Dnmt3b knockout mice (Okano et al. 1999). In this article, we summarize the results obtained from these experiments and discuss the role of de novo DNA methylation in the initiation of the two epigenetic phenomena.

Application of microarrays to the analysis of the inactivation status of human X-linked genes expressed in lymphocytes.

Abstract: Dosage compensation in mammalian females is achieved by the random inactivation of one X chromosome early in development; however, inactivation is not complete. In addition to a majority of pseudoautosomal loci, there are genes that are expressed from both the active and the inactive X chromosomes, and which are interspersed among other genes subject to regular dosage compensation. The patterns of X-linked gene expression in different tissues are of great significance for interpreting their impact on sex differences in development. We have examined the suitability and sensitivity of a microarray approach for determining the inactivation status of X-linked genes. Biotinylated cRNA from six female and six male lymphocyte samples were hybridised to Affymetrix HG-U133A microarrays. A total of 36 X-linked targets detected significantly higher levels of female transcripts, suggesting that these corresponded to sequences from loci that escaped, at least partly, from inactivation. These included genes for which previous experimental evidence, or circumstantial evidence, existed for their escape, and some novel candidates. Six of the targets were represented by more than one probe set, which gave independent support for the conclusions reached.

The Gene Sex-lethal of the Sciaridae Family (Order Diptera, Suborder Nematocera) and Its Phylogeny in Dipteran Insects

Abstract: This article reports the cloning and characterization of the gene homologous to Sex-lethal (Sxl) of Drosophila melanogaster from Sciara coprophila, Rhynchosciara americana, and Trichosia pubescens. This gene plays the key role in controlling sex determination and dosage compensation in D. melanogaster. The Sxl gene of the three species studied produces a single transcript encoding a single protein in both males and females. Comparison of the Sxl proteins of these Nematocera insects with those of the Brachycera showed their two RNA-binding domains (RBD) to be highly conserved, whereas significant variation was observed in both the N- and C-terminal domains. The great majority of nucleotide changes in the RBDs were synonymous, indicating that purifying selection is acting on them. In both sexes of the three Nematocera insects, the Sxl protein colocalized with transcription-active regions dependent on RNA polymerase II but not on RNA polymerase I. Together, these results indicate that Sxl does not appear to play a discriminatory role in the control of sex determination and dosage compensation in nematocerans. Thus, in the phylogenetic lineage that gave rise to the drosophilids, evolution coopted for the Sxl gene, modified it, and converted it into the key gene controlling sex determination and dosage compensation. At the same time, however, certain properties of the recruited ancestral Sxl gene were beneficial, and these are maintained in the evolved Sxl gene, allowing it to exert its sex-determining and dose compensation functions in Drosophila.

Sex chromosomes and male functions: where do new genes go?

Abstract: The position of a gene in the genome may have important consequences for its function. Therefore, when a new duplicate gene arises, its location may be critical in determining its fate. Our recent work in humans, mouse, and Drosophila provided a test by studying the patterns of duplication in sex chromosome evolution. We revealed a bias in the generation and recruitment of new gene copies involving the X chromosome that has been shaped largely by selection for male germline functions. The gene movement patterns we observed reflect an ongoing process as some of the new genes are very young while others were present before the divergence of humans and mouse. This suggests a continuing redistribution of male-related genes to achieve a more efficient allocation of male functions. This notion should be further tested in organisms employing other sex determination systems or in organisms differing in germline sex chromosome inactivation. It is likely that the selective forces that were detected in these studies are also acting on other types of duplicate genes. As a result, future work elucidating sex chromosome differentiation by other mutational mechanisms will shed light on this important process.

The Drosophila GAGA Factor Is Required for Dosage Compensation in Males and for the Formation of the Male-Specific-Lethal Complex Chromatin Entry Site at 12DE

Abstract: Drosophila melanogaster males have one X chromosome, while females have two. To compensate for the resulting disparity in X-linked gene expression between the two sexes, most genes from the male X chromosome are hyperactivated by a special dosage compensation system. Dosage compensation is achieved by a complex of at least six proteins and two noncoding RNAs that specifically associate with the male X. A central question is how the X chromosome is recognized. According to a current model, complexes initially assemble at ∼35 chromatin entry sites on the X and then spread bidirectionally along the chromosome where they occupy hundreds of sites. Here, we report that mutations in Trithorax-like ( Trl ) lead to the loss of a single chromatin entry site on the X, male lethality, and mislocalization of dosage compensation complexes.