Fluorescence and Giemsa banding studies of the allocyclic X chromosome in embryonic and adult mouse cells

DNA methylation is required to maintain both DNA replication timing precision and 3D genome organization integrity

DNA replication timing and three-dimensional (3D) genome organization are associated with distinct epigenome patterns across large domains. However, whether alterations in the epigenome, in particular cancer-related DNA hypomethylation, affects higher-order levels of genome architecture is still unclear. Here, using Repli-Seq, single-cell Repli-Seq, and Hi-C, we show that genome-wide methylation loss is associated with both concordant loss of replication timing precision and deregulation of 3D genome organization. Notably, we find distinct disruption in 3D genome compartmentalization, striking gains in cell-to-cell replication timing heterogeneity and loss of allelic replication timing in cancer hypomethylation models, potentially through the gene deregulation of DNA replication and genome organization pathways. Finally, we identify ectopic H3K4me3-H3K9me3 domains from across large hypomethylated domains, where late replication is maintained, which we purport serves to protect against catastrophic genome reorganization and aberrant gene transcription. Our results highlight a potential role for the methylome in the maintenance of 3D genome regulation.

Achieving singularity in mammalian odorant receptor gene choice

Odor discrimination requires differential expression of odor detectors. In fact, olfactory input to the brain is organized in units (glomeruli) innervated only by olfactory sensory neurons that express the same odorant receptor (OR). Therefore, discriminatory capacity is maximized if each sensory neuron expresses only one allele of a single OR gene, a postulate sometimes canonized as the "one neuron-one receptor rule." OR gene choice appears to result from a hierarchy of processes: differential availability of the alleles of each OR gene, zonal exclusion (or selection), OR gene switching during the initiation of OR gene transcription, and OR-dependent feedback to solidify the choice of one OR gene. The mechanisms underlying these processes are poorly understood, though a few elements are known or suspected. For example, the mechanism of activation of OR gene transcription appears to work in part through a few homeobox transcription factors (Emx2, and perhaps Lhx2) and the Ebf family of transcription factors. Further insights will probably come from several directions, but a promising hypothesis is that epigenetic mechanisms contribute to all levels of the hierarchical control of OR gene expression, especially the repressive events that seem to be necessary to achieve the singularity of OR gene choice.

CpG islands in human X-inactivation

Sequence comparison analyses have been carried out for 19 genes escaping X-inactivation versus 73 genes subject to X-inactivation, and 100 randomly chosen X chromosome genes versus 100 randomly chosen autosomal genes. The coding sequence of the genes and their upstream and downstream flanking sequences were investigated using a series of windows (1 kb, 2 kb, 5 kb, 10 kb and 100 kb). No significant difference in number of LINE-L1 elements was observed in genes escaping X-inactivation compared to genes subject to X-inactivation. This result, therefore, does not support the suggestion that lack of LINE repeat elements is a key factor for genes escaping X-inactivation. However, significantly reduced numbers of CpG islands and SINE MIR elements were found to be associated with genes escaping X-inactivation. Compared to genes known to be inactivated, genes escaping X-inactivation were observed to have fewer CpG islands, particularly within the 2 kb upstream flanking sequence close to the coding region. The results suggest that CpG islands may play a role in the process of X-inactivation by providing sufficient DNA methylation targets for the maintenance of X-inactivation. Lack of CpG islands may be a major reason for genes escaping X-inactivation regulation.

X-chromosome inactivation: lessons from transgenic mice

X-chromosome inactivation (XCI) is the process by which mammals perform dosage compensation of X-linked gene products between XY males and XX females, resulting in the transcriptional silencing of all but one X chromosome per diploid cell. XCI involves counting the X chromosomes in a cell, randomly choosing those to be inactivated, spreading the inactivation signal in cis throughout the chromosome, and maintaining the inactive state of those X chromosomes during cell divisions thereafter. How the cell performs all these tasks is a fascinating problem and, together with epigenetic inheritance, a basic cellular mechanism that remains to be fully understood. In this review, we describe recent experiments aimed at understanding the first events of XCI and propose a model for initiation of XCI.

Regulation of X-chromosome inactivation in development in mice and humans

Dosage compensation for X-linked genes in mammals is accomplished by inactivating one of the two X chromosomes in females. X-chromosome inactivation (XCI) occurs during development, coupled with cell differentiation. In somatic cells, XCI is random, whereas in extraembryonic tissues, XCI is imprinted in that the paternally inherited X chromosome is preferentially inactivated. Inactivation is initiated from an X-linked locus, the X-inactivation center (Xic), and inactivity spreads along the chromosome toward both ends. XCI is established by complex mechanisms, including DNA methylation, heterochromatinization, and late replication. Once established, inactivity is stably maintained in subsequent cell generations. The function of an X-linked regulatory gene, Xist, is critically involved in XCI. The Xist gene maps to the Xic, it is transcribed only from the inactive X chromosome, and the Xist RNA associates with the inactive X chromosome in the nucleus. Investigations with Xist-containing transgenes and with deletions of the Xist gene have shown that the Xist gene is required in cis for XCI. Regulation of XCI is therefore accomplished through regulation of Xist. Transcription of the Xist gene is itself regulated by DNA methylation. Hence, the differential methylation of the Xist gene observed in sperm and eggs and its recognition by protein binding constitute the most likely mechanism regulating imprinted preferential expression of the paternal allele in preimplantation embryos and imprinted paternal XCI in extraembryonic tissues. This article reviews the mechanisms underlying XCI and recent advances elucidating the functions of the Xist gene in mice and humans.

Stage-specific and cell type-specific aspects of genomic imprinting effects in mammals

Recent studies have revealed that maternal and paternal alleles of some imprinted genes are differentially expressed from the earliest time of expression, with virtually no expression from one of the two alleles, while for other imprinted genes the normally silent allele can be transcribed during early development. In addition, a number of imprinted genes manifest their imprints only in select tissues. These observations indicate that the marks that denote parental chromosome origin need not directly determine allele expression, but rather bias later epigenetic modifications toward a particular allele. Thus, factors expressed at specific stages or in specific cell types are required to silence one parental allele or another. Stage-dependent and tissue-specific epigenetic modifications include the progressive establishment of the mature adult parental allele-specific DNA methylation patterns. These changes resemble and may share a common mechanistic basis with other epigenetic modifications that occur during development. Understanding the mechanisms by which these post-fertilization epigenetic modifications are mediated and regulated will be essential for understanding how genomic imprinting leads to differences in parental allele expression.

Mechanistic and developmental aspects of genetic imprinting in mammals

Genetic imprinting in mammals allows the recognition and differential expression of maternal and paternal alleles of certain genes. Recent results from a number of laboratories indicate that, at least for some genes, gametic imprints, which must exist in order to mark chromosomes or genes as having been transmitted via sperm or ovum, are not by themselves sufficient to determine allele expression. Other postfertilization events are required, and these events are subject to both tissue-specific and developmental stage-specific regulation. Changes in imprinted gene methylation during preimplantation and fetal life indicate that the establishment of additional allele-specific modifications is likely to contribute to imprinted regulation. Disruptions in imprinting processes, loss of imprints, and loss of nonimprinted alleles through uniparental disomy are likely to contribute to a variety of developmental abnormalities and pathological conditions in both mice and humans.

Allelic inactivation regulates olfactory receptor gene expression

We suggest a model in which a hierarchy of controls is exerted on the family of odorant receptor genes to assure that a sensory neuron expresses a single receptor from a family of 1000 genes. We propose that a cis-regulatory element directs the stochastic expression of only one gene from a large array of linked receptor genes. Moreover, only one allelic array encoding multiple receptor genes is active in an individual neuron. We demonstrate that in a neuron expressing a given receptor, expression derives exclusively from one allele. In addition, we observe that alleles encoding the odorant receptors are replicated asynchronously, a phenomenon consistently associated with allelic inactivation. This model, involving inactivation of one allelic array and cis control of the active array, provides a mechanism such that individual neurons express one or a small number of receptors.

Genetic imprinting in the mouse: implications for gene regulation

Genetic imprinting specifies a germline marking that subsequently results in the repression of one or other parental allele at some point in development. Genetic manipulations to generate maternal and paternal duplications of specific chromosome regions have been used to screen almost the entire mouse genome for evidence of imprinting. As a result, 15 imprinting effects involving 10 regions on 6 different chromosomes have been detected that range from early embryonic lethalities to various growth and developmental defects seen only after birth. Genes with important roles in development therefore appear to be involved. Diverse studies have identified four imprinted genes, all of which show monoallelic expression in some, but not necessarily all, tissues. A correlation with methylation is indicated but the pattern of methylation is not consistent for each of the genes; methylation is therefore unlikely to be the imprinting signal. Methods being used to identify further imprinted genes are summarized and some of the difficulties posed are indicated.

Parental imprinting on the mouse X chromosome: effects on the early development of X0, XXY and XXX embryos

To examine the effects of X-chromosome imprinting during early mouse embryogenesis, we attempted to produce XM0, XP0, XMXMY, XMXPY and XMXMXP (where XM and XP stand for the maternally and the paternally derived X chromosome, respectively) making use of mouse strains bearing the translocation Rb(X.2)2Ad and the inversion In(X)1H. Unlike XMXPY embryos, XMXMY and XMXMXP conceptuses suffered from severe growth retardation or abnormal development characterized by deficient extra-embryonic structures at 6.5-7.5 days post coitum (dpc). A cytogenetic study suggested that two XM chromosomes remaining active in certain nonepiblast cells were responsible for the serious developmental abnormality found in these embryos disomic for XM. Although matings involving females heterozygous for Rb(X.2)Ad hinted at the paucity of XP0 embryos relative to those having the complementary karyotype of XMXMXP, further study of embryos from matings between females heterozygous for In(X)1H and Rb2Ad males did not substantiate this observation. Thus, the extensive peri-implantation loss of XP0 embryos shown by Hunt (1991) may be confined to X0 mothers. Taken together, this study failed to reveal a parentally imprinted X-linked gene essential for early mouse embryogenesis other than the one most probably corresponding to the X-chromosome inactivation centre.

Allele-specific replication timing of imprinted gene regions

Several lines of evidence suggest that the paternal and maternal genomes may have different expression patterns in the developing organism and this has been confirmed by the identification of endogenous genes that are parentally imprinted in the mouse. Little is known about the precise mechanisms involved in the process, but structural differences between the two alleles must somehow provide cis-acting signals for directing parental-specific transcription. Cell-cycle replication time is one parameter that has been shown to be associated with both tissue-specific gene expression and the allele-specific transcription patterns of the X chromosomes in female cells. For this reason we have examined the replication timing patterns for the chromosomal regions containing the imprinted genes Igf2, Igf2r, H19 and Snrpn in the mouse. At all of these sites, and their corresponding positions in the human genome, the two homologous alleles replicate asynchronously and it is always the paternal allele that is early-replicating. Thus imprinted genes appear to be embedded in large DNA domains with differential replication patterns, which may provide a structural imprint for parental identity.

Early development and X-chromosome inactivation in mouse parthenogenetic embryos

Early development and X-chromosome inactivation were studied in ethanol-induced mouse parthenogenones. About 24% of oocytes transferred to 0.5-day pseudopregnant recipients successfully implanted. However, only 49%, 20%, and 16% of implanted parthenogenones survived 5, 6, and 7 days later, respectively. Abnormal development was evident in every parthenogenone as early as 5 days after activation with the degenerating polar trophectoderm. These embryos were destined to become either small disorganized embryos or embryonic ectoderm vesicles bounded by the visceral endoderm. Only 2 of 51 representative 6- to 8-day parthenogenones sectioned had morphology of the normal egg cylinder, although growth retardation was evident. Spontaneous LT/Sv parthenogenones shared similar morphological features. In late blastocysts, the frequency of cells with an apparently inactivated X chromosome was lower in parthenogenones than in fertilized embryos. The failure of X-inactivation in the trophectoderm seems to contribute to the defective development of parthenogenones.

The X chromosome in development in mouse and man

In mammals, dosage compensation for X-linked genes between males and females is achieved by the inactivation of one of the X chromosomes in females. The inactivation event occurs early in development in all cells of the female mouse embryo and is stable and heritable in somatic cells. However, in the primordial germ cells, reactivation occurs around the time of meiosis. Owing to random inactivation in somatic cells, all female mice and humans are mosaic for X-linked gene function. Variable mosaicism can result in expression of disease in human females heterozygous for an X-linked gene defect. In the extra-embryonic lineages of female mouse embryos, and in the somatic cells of female marsupials, the paternally inherited X chromosome is preferentially inactivated. The X chromosomes in the egg and sperm must be differentially marked or imprinted, so that they are distinguished by the inactivation mechanism in these tissues. Initiation of inactivation of an entire X chromosome appears to spread from a single X-inactivation centre and may involve the recently discovered gene, XIST, which is expressed only from the inactive X chromosome. The maintenance of inactivation of certain household genes on the inactive X chromosome involves methylation of CpG islands in their 5' regions. Critical CpG sites are methylated at, or very close to, the time of inactivation in development. The mouse and the human X chromosomes carry the same genes but their arrangement is different and there are some genes in the pairing segment and elsewhere on the human X chromosome which can escape inactivation. Regions of homology between the mouse and human X chromosomes allow prediction of the map positions of homologous genes and provide mouse models of genetic disease in the human.

Fetal anomalies produced subsequent to treatment of zygotes with ethylene oxide or ethyl methanesulfonate are not likely due to the usual genetic causes

Earlier studies in this laboratory revealed that ethylene oxide (EtO) or ethyl methanesulfonate (EMS) induced high frequencies of midgestation and late fetal deaths, and of malformations among some of the surviving fetuses, when female mice were exposed at the time of fertilization of their eggs or during the early pronuclear stage of the zygote. Effects of the two mutagens are virtually identical. Thus, in investigating the mechanisms responsible for the dramatic effects in the early pronuclear zygotes, the two compounds were used interchangeably in the experiments. First, a reciprocal zygote-transfer study was conducted in order to determine whether the effect is directly on the zygotes or indirectly through maternal toxicity. And second, cytogenetic analyses of pronuclear metaphases, early cleavage embryos, and midgestation fetuses were carried out. The zygote transplantation experiment rules out maternal toxicity as a factor in the fetal maldevelopment. Together with the strict stage specificity observed in the earlier studies, this result points to a genetic cause for the abnormalities. However, the cytogenetic studies failed to show structural or numerical chromosome aberrations. Since intragenic base changes and deletions may also be ruled out, it appears that the lesions in question induced in zygotes by the two mutagens are different from conventional ones and, therefore, could be a novel one in experimental mammalian mutagenesis. Alternatively, the mechanism could involve a non-mutational 'imprinting' process that caused changes in gene expression.

5-aza-C-induced changes in the time of replication of the X chromosomes of Microtus agrestis are followed by non-random reversion to a late pattern of replication

Treatment with 5-azacytidine (5-aza-C) causes an advance in the time of replication and enhances the DNase-I sensitivity of the inactive X chromosome in Gerbillus gerbilllus fibroblasts. We found that these changes were not stably inherited and upon removal of the drug the cells reverted to the original state of one active and one inactive X chromosome. In order to determine whether this reversion was random, we used a cell line of female Microtus agrestis fibroblasts in which the two X chromosomes are morphologically distinguishable. In this work we show that the reversion to a late pattern of replication is not random, and the originally late replicating X chromosome is preferentially "reinactivated", suggesting an imprinting-like marking of one or both X chromosomes. The changes in the replication pattern of the X chromosome were associated with changes in total DNA methylation. Double treatment of cells with 5-aza-C did not alter this pattern of euchromatin activation and reinactivation. A dramatic advance in the time of replication of the entire X linked constitutive heterochromatin (XCH) region was however, observed in the doubly treated cells. This change in the replication timing of the XCH occurred in both X chromosomes and was independent of the changes observed in the euchromatic region. These observations suggest the existence of at least two independent regulatory sites which control the timing of replication of two large chromosomal regions.

Methylation of the Hprt gene on the inactive X occurs after chromosome inactivation

DNA sequences have previously been identified in the first intron of the mouse Hprt gene that are methylated on the inactive but not the active X chromosome. The temporal relationship between methylation of these sequences and X-inactivation was studied in teratocarcinoma cells and postimplantation mouse embryos: the sequences are unmethylated prior to X-inactivation and do not become methylated on the inactive X in most fetal cells until several days postinactivation. Such inactive X-specific methylation occurs in a significantly smaller proportion of the cells in the extra-embryonic tissues, yolk sac mesoderm and endoderm, than in the fetus. These data suggest that the inactive X-specific methylation of sequences such as those in the first intron of the Hprt gene does not play any role in the primary events of X-inactivation, but may function as part of a secondary, tissue-specific mechanism for maintaining the inactive state.

DNA hypomethylation causes an increase in DNase-I sensitivity and an advance in the time of replication of the entire inactive X chromosome

We have examined the effect of 5-azacytidine (5-aza-C) induced hypomethylation of DNA on the time of replication and DNase I sensitivity of the X chromosomes of female Gerbillus gerbillus (rodent) lung fibroblast cells. Using in situ nick translation to visualise the potential state of activity of large regions of metaphase chromosomes we show that 5-aza-C causes a dramatic increase in the DNase-I sensitivity of the entire inactive X chromosome of female G. gerbillus cells and this increase in nuclease sensitivity correlates with a large shift in the time of replication of the inactive X chromosome from late S phase to early S phase. These effects of 5-aza-C on the inactive X chromosome are associated with a 15% decrease in DNA methylation. Our results indicate that DNA methylation concomitantly affects both the time of replication and the chromatin conformation of the inactive X chromosome.

Allocyclic early replicating X chromosome in mice: genetic inactivity and shift into a late replicator in early embrogenesis

The allocyclic X chromosome in early female mouse embryos undergoes DNA replication either late or early in the S phase. Earlier studies indicated that the early-replicating X chromosome is restricted to the trophectoderm and primitive endoderm cell lineages in which the allocyclic X is almost exclusively paternal in origin. There has been, however, no compelling evidence for the genetic inactivity of the early-replicating X chromosome and a shift from early to late replication or vice versa. The present study employing a combination of 3H-thymidine autoradiography and BrdU labeling-acridine orange fluorescence staining in day-6 female mouse embryos found that the early-replicating X chromosome can change directly into a late-replicating one. The activity state of the early-replicating X chromosome was examined by electrophoretic determination of the X linked enzyme, phosphoglycerate kinase (PGK-1), in tissues isolated from 6.0-day and day-8.5 Pgk-1a/Pgk-1b embryos. Only the maternally derived Pgk-1 allele was expressed in the proximal endoderm and extraembryonic ectoderm of 6.0-day and the chorion of 8.5-day embryos. Thus, the early-replicating, paternally derived X chromosome found in about 70%-80% of the cells in these tissues seems to be repressed like the late-replicating one.

Regional and temporal changes in the pattern of X-chromosome replication during the early post-implantation development of the female mouse

We have made a detailed study of the X-chromosome replication pattern during the period when X-inactivation is occurring in the mouse embryo. Our observations show unequivocal regionalization of the embryo with respect to the temporal X-chromosome. The switch from isocyclic to allocyclic replication occurs in the embryonic ectoderm at approximately 6 days of development and is random with respect to parental origin of the X-chromosome. In the extra-embryonic tissues, however, the switch to allocyclic replication has apparently occurred prior to 5.3 days of development and almost exclusively involves the paternally-derived X-chromosome. Since these findings are consistent with results obtained in biochemical studies of X-chromosome activity in female embryos, we conclude that there is a close temporal relationship between the cytogenetic and biochemical manifestations of the X-inactivation process. In addition, we have observed a pattern of early paternal X-chromosome replication, transitory in some cases, that is unique to extra-embryonic tissues. These results suggest that there may be some differences in the mechanism by which X-inactivation occurs in the extra-embryonic tissues as compared with the embryonic ectoderm.

Cytogenetic studies in a selected group of mentally retarded children

Chromosomal abnormalities are an important cause of mental retardation. We studied the frequency of karyotype abnormalities in 74 mentally retarded patients selected from 306 patients referred to our clinic. Giemsa-banding was done on all cases. Additional studies in abnormal cases included autoradiography and X and Y chromatin. Karyotype analyses and blood group (Xg and Duffy) studies were carried out in family members in some cases. Fourteen of these children had chromosomal abnormalities, seven sex chromosomal, and seven had autosomal abnormalities. Three patients had 45,X and one had a 45,X/46,Xr(X) karyotype. Other sex chromosomal abnormalities were 46,XX/48,XXXX; 48,XXXY/49,XXXXY; and 48,XXYY. Autosomal abnormalities were 46,XX,1q-; 46,XY,2q-; 46,XY,5p-; 46,XY,dup(5p); 45,XX,t(13,14); and 46,XY,17p-. This is the first report from India of cytogenetic abnormalities in idiopathic mental retardation. The chromosomal studies in these patients help not only in accurate diagnosis, proper prognosis, and genetic counseling but also in gene localization and in the study of the origin of X-chromosome abnormalities.

Primary and secondary nonrandom X chromosome inactivation in early female mouse embryos carrying Searle's translocation T(X; 16)16H

By means of a cytological technique involving 5-bromodeoxyuridine, acridine orange, and fluorescence microscopy, the asynchronously replicating, hence genetically inactivated, X chromosome was identified in 6- to 8-day embryos from female mice heterozygous for Searle's translocation T(X;16)16H (abbreviated as T16H) mated with either karyotypically normal males or males carrying Cattanach's translocation T(X;7)1Ct in order to analyse the way in which the total inactivation of the normal X is achieved in adult T16H heterozygotes. Embryos examined included 9 Xn/X(7);16/16, 3 X 16/Xn;16x/16, 12 X16/X(7); 16x/16, 5 X16/Xn;16/16, 8 X16/X(7); 16/16 and 2 Xn/Y; 16x/16/16. In these notations X16, 16x, X(7) and Xn represent Searle's X with the centromeric segment of the X, Searle's X with the centomeric segment of chromosome 16, Cattanach's X with insertion of a chromosome 7 segment, and normal X, respectively. The X(7) exerted no apparent effect upon embryonic development up to the 8th day of gestation and X chromosome inactivation. -- The asynchronously replicating X was the Xn in X16/Xn;16x/16 and X(7) in X16/X(7);16x/16 embryos except a small number of cells on day 6 (13/493) and on day 7 (1/886) in which almost the entire 16x replicated asynchronously. The X16, on the other hand, never showed replication asynchrony. That the X16 is indeed unable to become inactivated was indicated by the observation that the X16 as well as Xn or X(7) did not replicate asynchronously in Xn/X16;16/16 and X16/X(7);16/16 embryos X16-inactive cell lines, if occurring, should have been genetically less unbalanced than any other cell line in such embryos. It is highly likely therefore that the ultimate inactivation pattern in T16H heterozygotes has been accomplished by (1) the inability of the X16 to become inactive; (2) inactivation in favor of the Xn; and (3) rapid elimination of 16x-inactive cells. Severe growth retardatin and early death of X16/Xn;16/16 and X16/X(7);16/16 embryos having no inactive X suggested that functional X disomy is detrimental to embryogenesis. These embryos further indicated that the concurrence of at least two X chromosomal loci separated by the T16H breakpoint is necessary for the homologous X chromosome becoming inactivated.

A possible active segment on the inactive human X chromosome

An idic(Xp--) in which the two X chromosomes are attached short arm to short arm, and which thus has two b regions (the Q-dark segment next to the centromere on Xp) between the inactivation centers, assumed to be situated on the Q-dark region next to the centromere on Xq, showed 63.8% bipartite Barr bodies as compared with 22.2% formed by idic(Xq--). In addition, the mean distance of the two parts of the Barr bodies in the fibroblasts of a patient with idic(Xp--) is significantly greater than in the cases with one or no b region. Contrary to the other patients with abnormal X chromosomes, the buccal cells of a woman idic(Xp--) showed a number of bipartite Barr bodies. -- To explain these observations we have put forward the hypothesis that the b region on the Xp always remains active and thus, when the rest of the chromosome forms a Barr body, this segment is extended, allowing the two parts of the X chromatin to get farther apart and at the same time increasing the percentage of bipartite bodies.

Stability of X chromosome differentiation in mouse embryos. Reversal may not be responsible for the extreme X-inactivation mosaicism in extraembryonic membranes

By means of a double labeling method with H3-thymidine and 5-bromodeoxyuridine, it was found that the X chromosome showed no sign of change from an allocyclic to an isocyclic state, or vice vers in 6.5- and 7.5-day mouse embryos. Thus, reversal of allocycly may not account for the predominance of cells with the paternally derived X chromosome inactive in the yolk sac and the chorion of the mouse embryo.

Is the centromeric heterochromatin of Mus musculus late replicating?

Quantitative analysis of DNA replication in embryonic cultures of C57BL/6J mice was carried out, using autoradiography after tritiated thymidine incorporation. The centromeric regions are late-replicating.

Mechanisms and evolutionary origins of variable X-chromosome activity in mammals

The X-chromosome of mammals is remarkable for its variable genetic activity. In somatic cells only a single X-chromosome is active, no matter how many are present, thus providing a dosage compensation mechanism by which males and females effectively have the same gene dosage of X-linked genes. In germ cells, however, it appears that all X-chromosomes present are active. Female germ cells require the presence of two X-chromosomes for normal survival, whereas male germ cells die if they have more than one X-chromosome. This system is found in all eutherian mammals and in marsupials, but is not known in any other animal group. In marsupials the X-chromosome derived from the father seems to be preferentially inactivated, whereas in eutherian mammals that from either parent may be so in different cells of the same animal. The differentiation of a particular X-chromosome as active or inactive is initiated in early embryogeny, and thereafter maintained through all further cell divisions in that individual. The mechanisms by which this is achieved are of great interest in relation to genetic control mechanisms in general. Various recent hypotheses concerning these mechanisms are discussed.