Showing papers on "Dosage compensation published in 2000"
TL;DR: It is established that O(2) levels regulate cell fate determination in vivo and that HIF is essential for mammalian placentation and that hypoxia promotes the in vitro differentiation of trophoblast stem cells into spongiotrophoblasts as opposed to giant cells.
Abstract: A two-fold difference in X chromosome dosage distinguishes males (XO) and hermaphrodites (XX) of the nematode Caenorhabditis elegans at the beginning of embryogenesis. As adults, individuals of the two sexes differ in size, anatomy, and behavior. About one-third of their somatic cells express sex-specific characteristics. How the X chromosome to autosome (X : A) ratio determines sex has been the subject of detailed genetic investigation (for review, see Meyer 1997). These studies have defined a regulatory pathway that transduces information about the X : A ratio to a gene known as tra-1.
In all somatic tissues, tra-1 activity is necessary and sufficient to promote female differentiation (Hodgkin 1987). Inhibition of tra-1 activity results in male development. The tra-1 gene encodes a zinc finger transcription factor, TRA-1A, that is related to Drosophila Ci and the Gli proteins of vertebrates (Zarkower and Hodgkin 1992). Few targets of TRA-1A have been identified, but in at least two cell types, TRA-1A represses genes that would otherwise cause adoption of male-specific fates. The survival of the HSN neurons, which are required for egg laying in hermaphrodites, depends on the repression of the egl-1 gene by TRA-1A (Conradt and Horvitz 1999). Similarly, vitellogenin synthesis by intestinal cells in the hermaphrodite requires that TRA-1A repress the mab-3 gene (Yi et al. 2000).
The role of tra-1 in the germ line is less well understood. Mutants lacking tra-1 activity, whether XX or XO, often exhibit limited spermatogenesis followed by oogenesis, suggesting that tra-1 is needed to sustain spermatogenesis (Schedl et al. 1989). On the other hand, strong gain-of-function alleles of tra-1 completely feminize the germ line, indicating that unregulated tra-1 activity can suppress spermatogenesis (Hodgkin 1980; 1987). Recent observations suggesting that TRA-1A may serve as both an activator and a repressor of the fog-3 gene, which is required for spermatogenesis, reinforce the idea that the function of tra-1 in the germ line is complex (Chen and Ellis 2000).
Negative regulation plays a prominent role in C. elegans sex determination (Fig. (Fig.1A).1A). Thus, male development in XO animals requires the inhibition of tra-1 activity by three fem genes (Doniach and Hodgkin 1984; Kimble et al. 1984; Hodgkin 1986). In XX animals, the tra-2 and tra-3 genes indirectly activate tra-1 by inhibiting fem activity. Support for this model comes from the observation that mutational inactivation of any of the fem genes renders tra-2 and tra-3 dispensable for female development. For example, both fem-1 mutants and tra-2; fem-1 double mutants develop as true females as a result of unregulated tra-1 activity (Doniach and Hodgkin 1984).
Figure 1
Model of somatic sex determination in C. elegans. (A) Genetic pathway regulating somatic sex determination. Barred lines indicate negative interactions, and arrows indicate positive interactions. The X/A ratio controls X chromosome dosage compensation ...
The major product of tra-2 is a large membrane protein known as TRA-2A (Fig. (Fig.1B)1B) (Kuwabara et al. 1992). A direct interaction between the intracellular domain of TRA-2A and FEM-3 inhibits the masculinizing activity of the FEM proteins, and although the mechanism of inhibition remains to be determined, this interaction would seem sufficient to explain the feminizing role of TRA-2A (Mehra et al. 1999). In support of this idea, overproduction of the intracellular domain of TRA-2A as a soluble protein in the somatic tissues of XO animals is strongly feminizing (Kuwabara and Kimble 1995; Mehra et al. 1999).
A 1.8 kb mRNA specific to the hermaphrodite germ line can encode a second TRA-2 protein known as TRA-2B, which is equivalent to the intracellular domain of TRA-2A (Okkema and Kimble 1991; Kuwabara et al. 1998). Genetic evidence suggests that TRA-2B may limit the extent of spermatogenesis in the hermaphrodite. Because TRA-2B includes the entire FEM-3-binding domain of TRA-2A, it is reasonable to expect that TRA-2B might also act by binding to and inhibiting FEM-3.
Although the mRNA encoding TRA-2B is restricted to the hermaphrodite germ line, a similar soluble protein might be produced in other tissues by cleavage of TRA-2A. The tra-3 gene, which acts at the same level in the sex-determining pathway as tra-2, encodes an atypical calpain protease (Barnes and Hodgkin 1996). When the two proteins are expressed together in insect cells, TRA-3 can cleave TRA-2A to release a fragment that includes part of the intracellular domain of TRA-2A (Sokol and Kuwabara 2000). This observation led to a suggestion that tra-3 might fulfill its role in sex determination by producing a soluble, FEM-3-binding fragment of TRA-2A. In this paper, we will use TRA-2c to refer to the intracellular domain of TRA-2A, whether it originates as part of TRA-2A or as TRA-2B.
Interaction with FEM-3 is the only effector function of TRA-2 that has been defined to date. FEM-3 binding does not require the C-terminal 200 amino acids of TRA-2c, but genetic evidence suggests that this region is nevertheless important for TRA-2 activity. Doniach (1986) identified tra-2 alleles that cause germ line feminization, a phenotype associated with increased tra-2 activity. Some of these alleles, now known as tra-2(mx) alleles, are unusual in that although they feminize the XX germ line, they appear to reduce tra-2 activity in somatic tissues. All of the tra-2(mx) alleles carry missense mutations that alter the sequence of a 22-amino acid region near the C-terminal of TRA-2c, outside the FEM-3-binding domain (Kuwabara et al. 1998). It has been suggested that this “MX region” might represent a domain required for interaction with a negative regulator of tra-2 in the germ line.
Here we report that TRA-2 directly interacts with TRA-1A and that the MX region of TRA-2 is critical for the interaction. The somatic effects of tra-2(mx) mutations and the results of overexpression experiments lead us to suggest that TRA-2 may promote somatic female development not only by inhibiting FEM-3, but also by directly stimulating the activity of TRA-1A. Mutations that alter either TRA-2 or TRA-1A so as to disrupt their interaction feminize the germ line. This observation raises the conundrum of why an interaction that enhances the feminizing activity of both proteins in somatic tissues should appear to have a role in hermaphrodite spermatogenesis.
413 citations
TL;DR: It is shown that association of MOF with the male X chromosome depends on its interaction with RNA, and MOF specifically binds through its chromodomain to roX2 RNA in vivo.
Abstract: In Drosophila, compensation for the reduced dosage of genes located on the single male X chromosome involves doubling their expression in relation to their counterparts on female X chromosomes1. Dosage compensation is an epigenetic process involving the specific acetylation of histone H4 at lysine 16 by the histone acetyltransferase MOF2,3,4,5. Although MOF is expressed in both sexes, it only associates with the X chromosome in males. Its absence causes male-specific lethality6. MOF is part of a chromosome-associated complex comprising male-specific lethal (MSL) proteins and at least one non-coding roX RNA7. How MOF is integrated into the dosage compensation complex is unknown. Here we show that association of MOF with the male X chromosome depends on its interaction with RNA. MOF specifically binds through its chromodomain to roX2 RNA in vivo. In vitro analyses of the MOF and MSL-3 chromodomains indicate that these chromodomains may function as RNA interaction modules. Their interaction with non-coding RNA may target regulators to specific chromosomal sites.
370 citations
TL;DR: It is demonstrated that the MSL complex is responsible for the specific chromatin modification characteristic of the X chromosome in Drosophila males.
Abstract: In Drosophila, dosage compensation-the equalization of most X-linked gene products in males and females-is achieved by a twofold enhancement of the level of transcription of the X chromosome in males relative to each X chromosome in females. A complex consisting of at least five gene products preferentially binds the X chromosome at numerous sites in males and results in a significant increase in the presence of a specific histone isoform, histone 4 acetylated at lysine 16. Recently, RNA transcripts (roX1 and roX2) encoded by two different genes have also been found associated with the X chromosome in males. We have partially purified a complex containing MSL1, -2, and -3, MOF, MLE, and roX2 RNA and demonstrated that it exclusively acetylates H4 at lysine 16 on nucleosomal substrates. These results demonstrate that the MSL complex is responsible for the specific chromatin modification characteristic of the X chromosome in Drosophila males.
330 citations
TL;DR: The results support a model in which MSL proteins assemble at specific chromatin entry sites (including the roX1 and roX2 genes); the ro X RNAs join the complex at their sites of synthesis; and complete complexes spread in cis to dosage compensate most genes on the X chromosome.
165 citations
TL;DR: It is demonstrated that JIL-1 colocalizes and physically interacts with male specific lethal (MSL) dosage compensation complex proteins, which strongly indicate Jil-1 associates with the MSL complex and further suggests J IL-1 functions in signal transduction pathways regulating chromatin structure.
Abstract: JIL-1 is a novel chromosomal kinase that is upregulated almost twofold on the male X chromosome in Drosophila. Here we demonstrate that JIL-1 colocalizes and physically interacts with male specific lethal (MSL) dosage compensation complex proteins. Furthermore, ectopic expression of the MSL complex directed by MSL2 in females causes a concomitant upregulation of JIL-1 to the female X that is abolished in msl mutants unable to assemble the complex. Thus, these results strongly indicate JIL-1 associates with the MSL complex and further suggests JIL-1 functions in signal transduction pathways regulating chromatin structure.
156 citations
TL;DR: The molecular phenotype of ICF cells is examined and several examples of extensive hypomethylation that are associated with advanced replication time, nuclease hypersensitivity and a variable escape from silencing for genes on the inactive X and Y chromosomes are reported.
Abstract: Chromosomal abnormalities associated with hypomethylation of classical satellite regions are characteristic for the ICF immunodeficiency syndrome. We, as well as others, have found that these effects derive from mutations in the DNMT3B DNA methyltransferase gene. Here we examine further the molecular phenotype of ICF cells and report several examples of extensive hypomethylation that are associated with advanced replication time, nuclease hypersensitivity and a variable escape from silencing for genes on the inactive X and Y chromosomes. Our analysis suggests that all genes on the inactive X chromosome may be extremely hypomethylated at their 5' CpG islands. Our studies of G6PD in one ICF female and SYBL1 in another ICF female provide the first examples of abnormal escape from X chromosome inactivation in untransformed human fibroblasts. XIST RNA localization is normal in these cells, arguing against an independent silencing role for this RNA in somatic cells. SYBL1 silencing is also disrupted on the Y chromosome in ICF male cells. Increased chromatin sensitivity to nuclease was found at all hypomethylated promoters examined, including those of silenced genes. The persistence of inactivation in these latter cases appears to depend critically on delayed replication of DNA because escape from silencing was only seen when replication was advanced to an active X-like pattern.
146 citations
TL;DR: Comparative genomic studies demonstrate that the trans‐acting factors (proteins and non‐coding RNAs) that have been shown to mediate dosage compensation are unrelated among the three lineages.
Abstract: Dosage compensation is the process by which the expression levels of sex-linked genes are altered in one sex to offset a difference in sex-chromosome number between females and males of a heterogametic species. Degeneration of a sex-limited chromosome to produce heterogamety is a common, perhaps unavoidable, feature of sex-chromosome evolution. Selective pressure to equalize sex-linked gene expression in the two sexes accompanies degeneration, thereby driving the evolution of dosage-compensation mechanisms. Studies of model species indicate that what appear to be very different mechanisms have evolved in different lineages: the male X chromosome is hypertranscribed in drosophilid flies, both hermaphrodite X chromosomes are downregulated in the nematode Caenorhabditis elegans, and one X is inactivated in mammalian females. Moreover, comparative genomic studies demonstrate that the trans-acting factors (proteins and non-coding RNAs) that have been shown to mediate dosage compensation are unrelated among the three lineages. Some tantalizing similarities in the fly and mammalian mechanisms, however, remain to be explained.
134 citations
TL;DR: This review discusses recent advances in understanding of how inactivation works, as well as the causes and clinical implications of deviations from random inactivation.
Abstract: X chromosome (X) inactivation is a remarkable biological process including the choice and cis-limited inactivation of one X, as well as the stable maintenance of this silencing by epigenetic chromatin alterations. The process results in females generally being mosaic for two populations of cells--one with each parental X active. In this review, we discuss recent advances in our understanding of how inactivation works, as well as the causes and clinical implications of deviations from random inactivation.
132 citations
TL;DR: This review summarizes current progress examining the roles these proteins, and the complexes they form, play in chromosome metabolism, and presents a twist in the SMC story, with the possibility of one SMC moonlighting in an unpredicted location.
128 citations
TL;DR: As unconventional RNAs are being encountered in novel epigenetic regulatory mechanisms, one striking feature is that the site of synthesis is critical to function.
125 citations
[...]
TL;DR: The recruitment of chromosome segregation proteins to the new task of regulating X-chromosome-wide gene expression points to the evolutionary origin of nematode dosage compensation.
TL;DR: A maternal imprint is set on the paternally derived X chromosome during oocyte growth, the maternal imprint serves to render the X(M) resistant to inactivation in the extraembryonic tissues and the X-M derived from an ng oocyte resembles a normal X(P).
Abstract: In mammals, X-chromosome inactivation occurs in all female cells, leaving only a single active X chromosome. This serves to equalise the dosage of X-linked genes in male and female cells. In the mouse, the paternally derived X chromosome (X(P)) is imprinted and preferentially inactivated in the extraembryonic tissues whereas in the embryonic tissues inactivation is random. To investigate how X(P) is chosen as an inactivated X chromosome in the extraembryonic cells, we have produced experimental embryos by serial nuclear transplantation from non-growing (ng) oocytes and fully grown (fg) oocytes, in which the X chromosomes are marked with (1) an X-linked lacZ reporter gene to assay X-chromosome activity, or (2) the Rb(X.9)6H translocation as a cytogenetic marker for studying replication timing. In the extraembryonic tissues of these ng/fg embryos, the maternal X chromosome (X(M)) derived from the ng oocyte was preferentially inactivated whereas that from the fg oocyte remained active. However, in the embryonic tissues, X inactivation was random. This suggests that (1) a maternal imprint is set on the X(M) during oocyte growth, (2) the maternal imprint serves to render the X(M) resistant to inactivation in the extraembryonic tissues and (3) the X(M) derived from an ng oocyte resembles a normal X(P).
TL;DR: It is demonstrated that macroH2A1.2 is present in the nuclei of germ cells starting first with localization that is largely, if not exclusively, to the developing XY-body in early pachytene spermatocytes.
TL;DR: It is concluded that a main role of MSL1 is to serve as the backbone for assembly of the MSL complex, and shows similarity to a region of mouse CBP, a transcription co‐activator.
Abstract: In male Drosophila, histone H4 acetylated at Lys16 is enriched on the X chromosome, and most X-linked genes are transcribed at a higher rate than in females (thus achieving dosage compensation). Five proteins, collectively called the MSLs, are required for dosage compensation and male viability. Here we show that one of these proteins, MSL1, interacts with three others, MSL2, MSL3 and MOF. The latter is a putative histone acetyl transferase. Overexpression of either the N- or C-terminal domain of MSL1 has dominant-negative effects, i.e. causes male-specific lethality. The lethality due to expression of the N-terminal domain is reduced if msl2 is co-overexpressed. MSL2 co-purifies over a FLAG affinity column with the tagged region of MSL1, and both MSL3 and MOF co-purify with the FLAG-tagged MSL1 C-terminal domain. Furthermore, the MSL1 C-terminal domain binds specifically to a GST-MOF fusion protein and co-immunoprecipitates with HA-tagged MSL3. The MSL1 C-terminal domain shows similarity to a region of mouse CBP, a transcription co-activator. We conclude that a main role of MSL1 is to serve as the backbone for assembly of the MSL complex.
TL;DR: MLE and MOF activities are necessary for complexes to access the various X chromosome sites and the role that histone H4 acetylation plays in this process is supported by observations that MOF overexpression leads to the ectopic association of the complex with autosomal sites.
Abstract: Dosage compensation in Drosophila is mediated by a multiprotein, RNA-containing complex that associates with the X chromosome at multiple sites. We have investigated the role that the enzymatic activities of two complex components, the histone acetyltransferase activity of MOF and the ATPase activity of MLE, may have in the targeting and association of the complex with the X chromosome. Here we report that MLE and MOF activities are necessary for complexes to access the various X chromosome sites. The role that histone H4 acetylation plays in this process is supported by our observations that MOF overexpression leads to the ectopic association of the complex with autosomal sites.
TL;DR: This finding supports the hypothesis that in fertilized female embryos, the maternal X chromosome remains active until the blastocyst stage because of a rigid imprint that prevents inactivation, whereas the paternal X chromosome is preferentially inactivated in extra-embryonic tissues owing to lack of such imprint.
Abstract: Using genetic and cytogenetic markers, we assessed early development and X-chromosome inactivation (X-inactivation) in XX mouse androgenones produced by pronuclear transfer. Contrary to the current view, XX androgenones are capable of surviving to embryonic day 7.5, achieving basically random X-inactivation in all tissues including those derived from the trophectoderm and primitive endoderm that are characterized by paternal X-activation in fertilized embryos. This finding supports the hypothesis that in fertilized female embryos, the maternal X chromosome remains active until the blastocyst stage because of a rigid imprint that prevents inactivation, whereas the paternal X chromosome is preferentially inactivated in extra-embryonic tissues owing to lack of such imprint. In spite of random X-inactivation in XX androgenones, FISH analyses revealed expression of stable Xist RNA from every X chromosome in XX and XY androgenonetic embryos from the four-cell to morula stage. Although the occurrence of inappropriate X-inactivation was further suggested by the finding that Xist continues ectopic expression in a proportion of cells from XX and XY androgenones at the blastocyst and the early egg cylinder stage, a replication banding study failed to provide positive evidence for inappropriate X-inactivation at E6. 5.
TL;DR: A number of systems that accurately compensate for sex-chromosome dosage have evolved independently: silencing a single X chromosome in female mammals, downregulating both X chromosomes in hermaphrodite Caenorhabditis elegans and upregulating theX chromosome in male Drosophila all equalize X-linked gene expression.
TL;DR: This study isolated, physically mapped, and determined the X inactivation status of a number of the orthologous mouse genes that correspond to this human ``escape domain'' and only the mouse Smcx gene has been found to escape inactivation in this region thus far, despite a highly conserved physical map between the two species.
Abstract: In females, most genes on the inactive X Chromosome (Chr) are transcriptionally silenced. However, several dozen genes have been identified in human that escape inactivation and are expressed from both the active and inactive X Chrs. Many of the genes that escape inactivation in human are subject to inactivation in mouse, raising questions concerning the mechanisms that govern expression from the inactive X Chr in the two species. In human, the existence of a cluster of genes in Xp11.21-p11.22 that escape inactivation suggests that control of X inactivation occurs at the level of chromosomal domains. In this study, we have isolated, physically mapped, and determined the X inactivation status of a number of the orthologous mouse genes that correspond to this human "escape domain". In contrast to human, only the mouse Smcx gene has been found to escape inactivation in this region thus far, despite a highly conserved physical map between the two species. Sequence analysis and functional characterization of the mouse Smcx promoter did not reveal any obvious unique features that would explain the difference in the behavior of this gene on the inactive X compared with other nearby genes. Possible mechanisms responsible for the differing inactivation status between genes in the escape domain in human Xp11. 21-p11.22 and the corresponding mouse region are discussed.
TL;DR: The results suggest that the well-known high micron nucleation and loss of the X chromosome in women's lymphocytes is the result of frequent distal lagging behind in anaphase and effective micronucleation of this chromosome.
Abstract: Summary Pancentromeric FISH and X-chromosome painting were used to characterize anaphase aberrations in 2,048 cultured lymphocytes from a healthy 62-year-old woman. Of 163 aberrant anaphases, 66.9% contained either chromosomes or their fragments that lagged behind. Characterization of 200 laggards showed that 49% were autosomes, 33.5% were autosomal fragments, and 17.5% were X chromosomes. The X chromosome represented one-fourth of all lagging chromosomes and was involved much more often than would be expected by chance (1/23). Labeling of the late-replicating inactive X chromosome with 5-bromo-2′-deoxyuridine revealed that both X homologues contributed equally to the laggards. Among 200 micronuclei examined from interphase cells, the proportion of the X chromosome (31%) and autosomal fragments (50%) was higher than among anaphase laggards, whereas autosomes were involved less often (19%). These findings may reflect either selection or the fact that lagging autosomes, which were more proximal to the poles than were lagging X chromosomes, were more frequently included within the main nucleus. Our results suggest that the well-known high micronucleation and loss of the X chromosome in women's lymphocytes is the result of frequent distal lagging behind in anaphase and effective micronucleation of this chromosome. This lagging appears to affect the inactive and active X chromosomes equally.
TL;DR: The hypothesis that two X chromosomes remaining active in the extraembryonic cell lineages due to the maternal imprinting explain the underdevelopment of extraembriesonic structures and hence early postimplantation death of DsXM embryos is supported.
Abstract: Mouse embryos having an additional maternally inherited X chromosome (XM) invariably die before midgestation with the deficient extraembryonic ectoderm of the polar trophectoderm lineage, whereas postnatal mice having an additional paternally inherited X chromosome (XP) survive beyond parturition. A cytogenetic study led us to hypothesize that abnormal development of such embryos disomic for XM (DsXM) is attributable to two doses of active XM chromosome in extraembryonic tissues. To test the validity of this hypothesis, we examined the initial X chromosome inactivation pattern in embryos at the blastocyst stage by means of replication banding method as well as RNA FISH detecting Xist transcripts. XP was the only asynchronously replicating X chromosome, if any, in XMXMXP blastocysts, and no such allocyclic X chromosome was ever detected in XMXMY blastocysts. In agreement with these findings, only one Xist paint signal was detected in 79% of XMXMXP cells, whereas no such signal was found in XMXMY embryos. Thus, the present study supports the hypothesis that two X chromosomes remaining active in the extraembryonic cell lineages due to the maternal imprinting explain the underdevelopment of extraembryonic structures and hence early postimplantation death of DsXM embryos.
TL;DR: In diploid species where sex determination involves heteromorphic sex chromosomes, a mechanism has evolved to compensate for gene-dosage differences in sex-linked genes between the sexes that relies on pioneer or novel components as well as on the presence of sequence-dependent target sites.
TL;DR: The severe hemophilia A in the proposita results from a de novo F8 gene frameshift-stop mutation on the paternally derived X chromosome, associated with a nonrandom pattern of inactivation of the maternallyderived X chromosome.
TL;DR: A novel class of noncoding RNAs that paint entire chromosomes are centrally involved in this process, and are key sites that target the X for dosage compensation.
TL;DR: The results indicate that androgenones may lack a factor that is expressed from the maternal genome and required for dosage compensation in preimplantation embryos, and indicates that early dosage compensation for the X chromosome may normally be reversible, thus providing flexibility to meet different developmental requirements of the embryonic and extraembryonic lineages.
Abstract: Diploid androgenetic mouse embryos, possessing two sets of paternally inherited chromosomes, and control fertilized embryos were used to examine the relative effects of X chromosome number and parental chromosome origin on androgenone viability and X-linked gene expression. A significant difference in efficiency of blastocyst formation was observed between XX and XY androgenones in some experiments, but this difference was not uniformly observed. Significant effects of both X chromosome number and parental origin on X-linked gene expression were observed. Male and female control embryos expressed the Xist RNA initially. This expression was followed by a preferential reduction in Xist RNA abundance in male embryos, indicating that dosage compensation for the X chromosome may normally require the downregulation of Xist RNA expression in male embryos, in conjunction with the production of stable Xist transcripts in female embryos. By the late blastocyst stage, XX control embryos expressed significantly more Xist RNA than did XY embryos. Unlike their normal counterparts, XX androgenones did not express significantly more Xist RNA than did XY androgenones at the late blastocyst stage. Androgenones exhibited severe repression of the Pgk1 gene, but during development to the late blastocyst stage Pgk1 mRNA expression increased in XX androgenones and decreased in XY androgenones. Thus, the initial repression of the Pgk1 gene in XX androgenones was lost as the Xist RNA declined in abundance, and this loss was correlated with a failure of XX androgenones to express significantly more Xist RNA than did XY androgenones. These results indicate that androgenones may lack a factor that is expressed from the maternal genome and required for dosage compensation in preimplantation embryos. The results also indicate that early dosage compensation in preimplantation embryos may normally be reversible, thus providing flexibility to meet different developmental requirements of the embryonic and extraembryonic lineages.
TL;DR: A novel RT-PCR-based approach for determining the inactivation status of X-linked genes is presented, and the potential for widespread application was shown by the successful demonstration of inactivation at the MAOA and HPRT loci using intronic polymorphisms.
Abstract: We present a novel RT-PCR-based approach for determining the inactivation status of X-linked genes. Using cDNA from cloned female cell lines in which only the maternal or paternally derived X chromosome is active, we are able to demonstrate expression from only one allele in genes known to be inactivated. Following reverse transcription, amplification across a polymorphism will yield a product from a single allele if the gene of interest is inactivated, and products from both alleles in a gene escaping inactivation. We have verified this approach using the human androgen receptor and FMR1 loci which have been shown to be subjected to normal inactivation. The potential for widespread application of this approach was shown by the successful demonstration of inactivation at the MAOA and HPRT loci using intronic polymorphisms.
TL;DR: incipient speciation, manifest as hybrid sterility when "varieties" are crossed, would appear at the earliest stage in the heterogametic sex, even in genera with homomorphic sex chromosomes (Haldane's rule for Hybrid sterility), and it has been proposed that Haldane’s rule for hybrid inviability needs differences in dosage compensation, so could not apply to generaWith homomorphicsex chromosomes.
TL;DR: The cloning and characterization in S. ocellaris of the gene homologous to maleless (mle) of D. melanogaster, which implements dosage compensation is reported, and it is proposed that different proteins in Drosophila and Sciara would implement dosage compensation.
Abstract: In Drosophila melanogaster and in Sciara ocellaris dosage compensation occurs by hypertranscription of the single male X chromosome. This article reports the cloning and characterization in S. ocellaris of the gene homologous to maleless (mle) of D. melanogaster, which implements dosage compensation. The Sciara mle gene produces a single transcript, encoding a helicase, which is present in both male and female larvae and adults and in testes and ovaries. Both Sciara and Drosophila MLE proteins are highly conserved. The affinity-purified antibody to D. melanogaster MLE recognizes the S. ocellaris MLE protein. In contrast to Drosophila polytene chromosomes, where MLE is preferentially associated with the male X chromosome, in Sciara MLE is found associated with all chromosomes. Anti-MLE staining of Drosophila postblastoderm male embryos revealed a single nuclear dot, whereas Sciara male and female embryos present multiple intranuclear staining spots. This expression pattern in Sciara is also observed before blastoderm stage, when dosage compensation is not yet set up. The affinity-purified antibodies against D. melanogaster MSL1, MSL3, and MOF proteins involved in dosage compensation also revealed no differences in the staining pattern between the X chromosome and the autosomes in both Sciara males and females. These results lead us to propose that different proteins in Drosophila and Sciara would implement dosage compensation.
TL;DR: Possession of a r(X) chromosome was associated with an increased risk of significant learning difficulties, and with associated behavioural maladjustment, compared with 45,X Turner females, and the proportion of cells in peripheral blood containing an inactivated r( X) chromosomes was negatively correlated with nonverbal IQ.
Abstract: We tested the cognitive abilities and educational attainments of 47 patients with a ring X chromosome, to evaluate the extent to which these variables correlated with failure of r(X) inactivation and with mosaicism. We found possession of a r(X) chromosome was associated with an increased risk of significant learning difficulties, and with associated behavioural maladjustment, compared with 45,X Turner females. Nearly a third had been educated outside mainstream schools. The proportion of cells in peripheral blood containing an inactivated r(X) chromosome was negatively correlated with nonverbal IQ. The parental origin of the normal chromosome did not appear to affect adjustment or abilities. In a minority of r(X) cases associated with mental retardation, there had been a failure to inactivate the ring, due to loss of the XIST locus. However, failure of X-inactivation was not necessarily associated with a severe phenotype. The degree of impairment in IQ depended on the size of the active ring, and hence was proportionate to the number of (as yet unidentified) genes whose functional disomy affected brain development and functioning.
TL;DR: Six new X-linked single-nucleotide polymorphisms are identified and the inactivation status of these genes are determined by examination of expression patterns in female cells previously demonstrated to have skewed inactivation, as well as by analysis of somatic cell hybrids retaining the inactive human X chromosome.
TL;DR: The dosage compensation complex was found to associate with a duplication of the right 30% of X, but the complex did not spread onto adjacent autosomal sequences, indicating that all the information required to specify X chromosome identity resides on the duplication and that the dosage compensation machinery can localize to a site distinct from the full-length hermaphrodite X chromosome.
Abstract: The dosage compensation machinery of Caenorhabditis elegans is targeted specifically to the X chromosomes of hermaphrodites (XX) to reduce gene expression by half. Many of the trans-acting factors that direct the dosage compensation machinery to X have been identified, but none of the proposed cis-acting X chromosome-recognition elements needed to recruit dosage compensation components have been found. To study X chromosome recognition, we explored whether portions of an X chromosome attached to an autosome are competent to bind the C. elegans dosage compensation complex (DCC). To do so, we devised a three-dimensional in situ approach that allowed us to compare the volume, position, and number of chromosomal and subchromosomal bodies bound by the dosage compensation machinery in wild-type XX nuclei and XX nuclei carrying an X duplication. The dosage compensation complex was found to associate with a duplication of the right 30% of X, but the complex did not spread onto adjacent autosomal sequences. This result indicates that all the information required to specify X chromosome identity resides on the duplication and that the dosage compensation machinery can localize to a site distinct from the full-length hermaphrodite X chromosome. In contrast, smaller duplications of other regions of X appeared to not support localization of the DCC. In a separate effort to identify cis-acting X recognition elements, we used a computational approach to analyze genomic DNA sequences for the presence of short motifs that were abundant and overrepresented on X relative to autosomes. Fourteen families of X-enriched motifs were discovered and mapped onto the X chromosome.