Evidence for two functional X chromosomes in human oocytes

Global Dosage Compensation Is Ubiquitous in Lepidoptera, but Counteracted by the Masculinization of the Z Chromosome

While chromosome-wide dosage compensation of the X chromosome has been found in many species, studies in ZW clades have indicated that compensation of the Z is more localized and/or incomplete. In the ZW Lepidoptera, some species show complete compensation of the Z chromosome, while others lack full equalization, but what drives these inconsistencies is unclear. Here, we compare patterns of male and female gene expression on the Z chromosome of two closely related butterfly species, Papilio xuthus and Papilio machaon, and in multiple tissues of two moths species, Plodia interpunctella and Bombyx mori, which were previously found to differ in the extent to which they equalize Z-linked gene expression between the sexes. We find that, while some species and tissues seem to have incomplete dosage compensation, this is in fact due to the accumulation of male-biased genes and the depletion of female-biased genes on the Z chromosome. Once this is accounted for, the Z chromosome is fully compensated in all four species, through the up-regulation of Z expression in females and in some cases additional down-regulation in males. We further find that both sex-biased genes and Z-linked genes have increased rates of expression divergence in this clade, and that this can lead to fast shifts in patterns of gene expression even between closely related species. Taken together, these results show that the uneven distribution of sex-biased genes on sex chromosomes can confound conclusions about dosage compensation and that Z chromosome-wide dosage compensation is not only possible but ubiquitous among Lepidoptera.

Balancing the dose in the mouse

Organisms that use a chromosomal basis of sex determination have a problem of gene inequality. In the mouse, this dimorphism is evident by the presence of two X-chromosomes in females, while males have a single X- and a single Y-chromosome. To balance this disparity, one of the two female X-chromosomes is transcriptionally silenced to neutralize the gene dose with the XY male. Dosage compensation in mammals is known as X-chromosome inactivation (XCI) and is a crucial early developmental process. XCI is an example of epigenetics: a phenotype resulting in changes on a chromosome without a change in nucleic acid sequence. Studies in mouse embryology and genetics have answered many questions about the process of balancing the dose. In this chapter, I highlight how the mouse dosage compensates the gene disparity between XX females and XY males in a crucial epigenetic process called X-chromosome inactivation (XCI).

Global analysis of X-chromosome dosage compensation

Background

Drosophila melanogaster females have two X chromosomes and two autosome sets (XX;AA), while males have a single X chromosome and two autosome sets (X;AA). Drosophila male somatic cells compensate for a single copy of the X chromosome by deploying male-specific-lethal (MSL) complexes that increase transcription from the X chromosome. Male germ cells lack MSL complexes, indicating that either germline X-chromosome dosage compensation is MSL-independent, or that germ cells do not carry out dosage compensation.

Results

To investigate whether dosage compensation occurs in germ cells, we directly assayed X-chromosome transcripts using DNA microarrays and show equivalent expression in XX;AA and X;AA germline tissues. In X;AA germ cells, expression from the single X chromosome is about twice that of a single autosome. This mechanism ensures balanced X-chromosome expression between the sexes and, more importantly, it ensures balanced expression between the single X chromosome and the autosome set. Oddly, the inactivation of an X chromosome in mammalian females reduces the effective X-chromosome dose and means that females face the same X-chromosome transcript deficiency as males. Contrary to most current dosage-compensation models, we also show increased X-chromosome expression in X;AA and XX;AA somatic cells of Caenorhabditis elegans and mice.

Conclusion

Drosophila germ cells compensate for X-chromosome dose. This occurs by equilibrating X-chromosome and autosome expression in X;AA cells. Increased expression of the X chromosome in X;AA individuals appears to be phylogenetically conserved.

Dosage compensation of the active X chromosome in mammals

Monosomy of the X chromosome owing to divergence between the sex chromosomes leads to dosage compensation mechanisms to restore balanced expression between the X and the autosomes. In Drosophila melanogaster, upregulation of the male X leads to dosage compensation. It has been hypothesized that mammals likewise upregulate their active X chromosome. Together with X inactivation, this mechanism would maintain balanced expression between the X chromosome and autosomes and between the sexes. Here, we show that doubling of the global expression level of the X chromosome leads to dosage compensation in somatic tissues from several mammalian species. X-linked genes are highly expressed in brain tissues, consistent with a role in cognitive functions. Furthermore, the X chromosome is expressed but not upregulated in spermatids and secondary oocytes, preserving balanced expression of the genome in these haploid cells. Upon fertilization, upregulation of the active X must occur to achieve the observed dosage compensation in early embryos.

Sex chromosomes, recombination, and chromatin conformation

We review what is known about the transcriptional inactivation and condensation of heteromorphic sex chromosomes in contrast to the activation of homomorphic sex chromosomes during meiotic prephase in animals. We relate these cytological and transcriptional features to the recombination status of the sex chromosomes. We propose that sex chromosome condensation is a meiotic adaptation to prevent the initiation of potentially damaging recombination events in nonhomologous regions of the X and Y chromosome.

Expression of Xist in mouse germ cells correlates with X–chromosome inactivation

Mammals compensate for different doses of X-chromosome-linked genes in male (XY) and female (XX) somatic cells by terminally inactivating all but one X chromosome in each cell. A transiently inactive X chromosome is also found in germ cells, specifically in premeiotic oogenic cells and in meiotic and postmeiotic spermatogenic cells. Here we show that the Xist gene, which is a expressed predominantly from the inactive X-chromosome in female somatic cells, is also expressed in germ cells of both sexes, but only at those stages when an inactive X chromosome is present. This suggests support for the putative role of Xist as a regulator of X-chromosome inactivation and suggest a common mechanism for the initiation and/or maintenance of X-chromosome inactivation in all cell types.

A potentially critical Hpa II site of the X chromosome-linked PGK1 gene is unmethylated prior to the onset of meiosis of human oogenic cells

Hpa II site H8 is in the CpG-rich 5' untranslated region of the human X chromosome-linked gene for phosphoglycerate kinase 1 (PGK1). It is the only Hpa II site in the CpG "island" whose methylation pattern is perfectly correlated with transcriptional silence of this gene. We measured DNA methylation at site H8 in fetal oogonia and oocytes and found, using a quantitative assay based on the polymerase chain reaction, that purified germ cells isolated by micromanipulation were unmethylated in 47-day to 110-day fetuses, whereas ovaries depleted of germ cells and non-ovary tissues were methylated. We conclude that site H8 is unmethylated in germ cells prior to the onset of meiosis and reactivation of the X chromosome.

Monosomy X and gonadal dysgenesis in a buffalo heifer (Bubalusbubalis)

Investigations on a 3-yr-old river buffalo heifer presenting anestrus revealed a chromosome make-up of 2n=49 in the lymphocyte cultures, compared with the 2n=50 characteristic of riverine buffalo. The missing chromosome was identified as one of the Xs by karyotypic analysis, and monosomy X was confirmed by C and G-banding techniques. Both ovaries of the heifer were underdeveloped, although the other components of the internal genitalia were normal. The phenotypic and karyotypic features confirmed this to be a case of ovarian dysgenesis with 49,XO karyotype similar to that of the Turner's syndrome in man and other mammals.

Sex difference in methylation of single-copy genes in human meiotic germ cells: implications for X chromosome inactivation, parental imprinting, and origin of CpG mutations

To determine the methylation status of female germ cells in reference to the programmed reversal of X chromosome inactivation in these cells, we examined human fetal ovaries at developmental stages from the time germ cells initiate meiosis to when they cease to synthesize DNA (8-21 weeks gestation). Using methylation-sensitive restriction enzymes, we analyzed 57 MspI sites (32 sites in the CpG islands, and 25 nonclustered sites) from five X-linked housekeeping genes (HPRT, G6PD, P3, PGK, and GLA) and two tissue specific genes (X-linked F9 and autosomal EPO). Methylation patterns were compared to those of male germ cells, sperm, and somatic tissues of both sexes. All 32 MspI sites in CpG islands were unmethylated in germ-cell fractions of fetal ovary and adult testes, which could explain the reversibility of X inactivation in these tissues. However, whereas male meiotic germ cells were extensively methylated outside the islands (in the body of genes) and the methylation patterns resembled those of most somatic tissues, none of the 25 nonclustered CpGs was methylated in DNA contributed by the germ-cell component of fetal ovaries. The presence of faint MspI-like fragments in HpaII digests of fetal testes as well as fetal ovary prior to the onset of meiosis suggests that DNA of primordial germ cells is unmethylated in both sexes. Our observations of meiotic germ cells suggest that the female germ cells remain unmethylated, but that methylation in male germ cells occurs postnatally, prior to or during the early stages of spermatogenesis. In any event, the striking sex difference in methylation status of endogenous single-copy genes in meiotic germ cells could provide a molecular basis for parental imprinting of the mammalian genome.

Studies of skin tumorigenesis in PGK mosaic mice: many promoter-independent papillomas and carcinomas do not develop from pre-existing promoter-dependent papillomas

The induction of mouse skin papillomas in two stages by initiation-promotion protocols has been studied extensively. On the basis of kinetics of induction and morphological observations of apparent progression to carcinomas, it has been argued that the papillomas sequentially advance to carcinomas through promoter-dependent and promoter-independent premalignant stages. Although this hypothesis has been widely accepted, the step-wise development of the same cell population from papilloma to carcinoma has not been established at the cellular level. We show here that some carcinomas which morphologically appear to result from progression of promoter-dependent papillomas do not develop from sequential progression of the same cell population as was found in the original papillomas. Other initiated cells also participate during the growth of these tumors and may replace the original papilloma cells. We also present evidence that some tumors develop directly as promoter-independent papillomas without going through an apparent promoter-dependent papilloma stage.

Reactivation of X-linked genes in human fibroblasts transformed by origin-defective SV40

To determine if expression of genes on the inactive X is inducible in human cells, we looked for reactivation events in a clone of fibroblasts transformed with origin-defective SV40. The karyotype of these cells was grossly heteroploid so that the aneuploidy associated with SV40 transformation occurs even in the absence of viral replication. This transformed clone, heterozygous for hypoxanthine phosphoribosyltransferase (HPRT), lacks HPRT activity, as the mutant allele is on the active X and the normal allele on the inactive X. Reactivation of the HPRT+ allele on the inactive X was observed at a frequency of 6 X 10(-5) per cell and increased approximately eightfold following treatment with the cytidine analogs 5-azacytidine (5azaC) and 5-azadeoxycytidine. The fact that spontaneous reactivation is detectable in some clones, but not all, suggests that the environment of the SV40-transformed cell, although not sufficient to induce generalized derepression, increases the frequency of rare reactivation events. The methylation pattern at the HPRT locus revealed transformation-associated alterations that may have predisposed these cells to reactivation events, spontaneous as well as 5azaC-induced.

Derepression of HPRT locus on inactive X chromosome of human lymphoblastoid cell line

Human XX lymphoblastoid cells with a deletion in the HPRT locus on the active X were exposed to HPRT clone pHPT32. HPRT+ isolates GPT3 and GPT5 lacked pHPT32 DNA, suggesting that their HPRT+ phenotype resulted from expression of a cellular gene. GPT3 mutated to thioguanine resistance at least 100 times more frequently than cells in which the expressed HPRT locus was on the active X. Most GPT3-derived HPRT- had lost one entire X chromosome, indicating that the HPRT+ phenotype of GPT3 resulted from derepression of the HPRT locus on its inactive X. Virtually unchanged G6PD and PGK activities and the presence of a late-replicating X in GPT3 suggest that derepression of the inactive X was not general. Eleven of the GPT3-derived mutants had a tiny centric remnant that may result from a frequently operative mechanism of X chromosome loss. The detection of partial or complete loss of an X by direct selection presents unusual opportunities for genotoxicity detection with human cells.

Biochemical and genetic properties of oligomeric structures: a general approach

The oligomers constituted by association of different subunits can exist under multiple forms. In the case of the genetically variable proteins, such a multiplicity leads to numerous questions (i) on the enumerations: what is the number of active forms when a given subunit can make the oligomer inactive, or when the subunits are encoded by s alleles; (ii) on the subunit effects on biochemical properties: how to estimate these effects, are they equal, are there interactions between subunits, etc. Theoretical methods for the study of such oligomeric structures are developed, which mainly rely on linear model techniques. Peculiar properties examined are Vmax and Km, but also the quantities of the various oligomers, which depend on their association law. This approach is extended to the oligomers composed of different sets of subunits, as are for example some enzymes. These aspects are discussed from numerous bibliographic examples, with special reference to molecular interactions (protein complementation or molecular heterosis). Otherwise the genetic application of this theoretical approach is presented: it is possible to consider a genotype as an oligomer of alleles, and thus to study their effects and their interactions, in the one-locus case as well as in the several-loci case. The relevance of this generalization is discussed in connection with two other concepts, the "sequence space" used in molecular evolution and the regression of the genotypic values on the number of alleles used in quantitative genetics.

Glucose 6-Phosphate dehydrogenase deficiency genetic, pathophysiological and therapeutic aspects

Glucose 6-phosphate dehydrogenase (G6PD) is the first enzyme of the pentose phosphate pathway. It exists in over 250 variants which are divided broadly into five classes on the basis of residual enzyme activity and clinical manifestations. The variants with reduced activity result in G6PD deficiency. This is inherited as an X-linked recessive disorder and occurs at a much higher frequency in the male than in the female. Most G6PD deficient individuals show no clinical abnormality under normal conditions, but acute hemolytic crisis may occur. Several nonhemolytic abnormalities occur as well in G6PD deficients at a higher frequency than in nondeficients. Several nonhemolytic abnormalities occur as well in G6PD deficients at a higher frequency than in nondeficients. The genetic, pathophysiologic, and therapeutic aspects of G6PD deficiency are presented; and the possibility of genetic counselling, care in blood banks, and benefits of education the G6PD deficients are discussed.

Developmental aspects of X chromosome inactivation in eutherian and metatherian mammals

The single active X principle has served for two decades as a focal point for research on the cyclic activation and inactivation of gene loci. Differences in X chromosome inactivation patterns of eutherian and marsupial mammals provide probes for investigating the mechanisms of the X inactivation process. In eutherian mammals, the X chromosome is inactivated early in meiotic prophase in males and remains inactive throughout the rest of spermatogenesis. During meiosis in females, the inactive X chromosome is activated so that both X chromosomes are active in oocytes. During the early cleavage divisions of female embryos, the paternally derived X is activated. It and the maternally derived X remain active until differentiation begins in early embryogenesis. At that time, the paternally derived X is inactivated in cells that give rise to extraembryonic membranes, whereas a random process determines which X chromosome is inactivated in cells that give rise to the embryo itself. Although less is known about developmental aspects of X inactivation in female marsupials, it is clear that the paternal X is preferentially inactive in postembryonic somatic cells. Furthermore, the paternal X is partially active at some loci in some cell types, indicating that it is not regulated as a single unit. The successful adaptation of a small (80-150 g), fecund marsupial to simple laboratory conditions now enables extensive experimentation on the large number of marsupials at various developmental stages. This capability, coupled with the application of newly developed cellular and molecular techniques to questions about X chromosome inactivation, shows great promise for advancing our understanding of the mechanisms that control the cyclic behavior of X chromosome activity.

Derepression of genes on the human inactive X chromosome: evidence for differences in locus-specific rates of derepression and rates of transfer of active and inactive genes after DNA-mediated transformation

Mouse-human hybrid cells that contained an inactive human X chromosome were treated with agents known to alter gene expression and to perturb DNA methylation. 5-Azacytidine greatly increased the rate of derepression of HPRT on the inactive X, while butyrate and dimethyl sulfoxide had smaller effects. Ethionine did not change the rate of derepression. Derepression of two other X-chromosomal loci, PGK and GPD, was also detected. The rate of derepression of PGK was 20-fold higher than the rate for HPRT. Derepression events at the two loci appeared to be independent. Hybrids expressing derepressed X-chromosomal genes had more variable levels of human enzyme activities when compared to control hybrids. HPRT+ clones did not appear after transfer of purified DNA from a cell hybrid containing an inactive human X into HPRT- recipients, but such clones did appear after transfer of DNA from derivative cells in which HPRT had been derepressed.

A stem-line model for cellular and chromosomal differentiation in early mouse-development

Differentiation in mouse embryo development is represented formally by means of a stem-line model in which: 1. Individual choices are structured so that only a fraction of the cells of a given population receive a signal to undergo a change of state that results in a departure from the stem line. 2. Development proceeds by a series of restrictions in potency of all the cells in the stem line. 3. The stem line harbours the germ cells. 4. The germ cells are returned to a state of totipotency by some event(s) leading to meiosis.

X chromosome reactivation in oocytes of Mus caroli

Mature mammalian oocytes have both of their X chromosomes active, while somatic cells from the same individual have one of their X chromosomes in an inactive state. We asked whether the X chromosomes of the germ cells never undergo inactivation in their ontogeny or whether inactivation of an X chromosome does occur but is followed by a subsequent reactivation event. Our approach has used an electrophoretic polymorphism for the X-linked enzyme glucose-6-phosphate dehydrogenase (G6PD) in the mouse species Mus caroli. G6PD is dimeric, and a heterodimer is produced in cells from heterozygous females if and only if both X chromosomes are active. Ovaries from heterozygous fetuses at different gestational ages were dissected and either studied cytologically or pressed between microscopy slides to obtain germ cell-rich and germ cell-poor preparations. No heterodimer band was detected on the 10th day of development in germ cell-rich preparations. On subsequent days, an increasingly intense heterodimer band was detected, which, by the 13th day, was approximately twice as intense as the corresponding homodimer bands. Consideration of (i) the G6PD activity per germ cell and per somatic cell and (ii) the percentage of germ cells in the germ cell-rich preparations indicated that a heterodimer band should have been visible on the 10th day had both X chromosomes been active. Cytological examinations showed that the earliest germ cells enter meiotic prophase on the eleventh day. These results demonstrate that oogonia have a single active X chromosome and that the inactive X chromosome is reactivated at or, more likely, shortly before entry into meiotic prophase.

Mammalian gonadal determination and gametogenesis

Although a relationship between the X and Y chromosomes and mammalian sexual development has long been recognized, a detailed understanding of this relation is still lacking. Recent advances in somatic cell genetics and recombinant DNA technology should provide the tools for solving this fundamental problem in developmental genetics.

A synopsis of the human Y chromosome

Phenotypic features and functions known to depend on the presence of the Y chromosome or the H-Y antigen are discussed in relation to structural anomalies of the Y chromosome and other abnormalities of sexual and somatic development. Recent knowledge about molecular organization of constitutive heterochromatin in relation to the human Y is presented. An attempt is made at assigning different functions, genes and DNA sequences to different regions of the Y chromosome.

DNA repair defects and chromosome instability disorders

Xeroderma pigmentosum (XP), Fanconi anaemia (FA), ataxia telangiectasia (AT) and Bloom disease (BS) are four rare autosomal recessive disorders in which there is defective DNA repair and/or chromosome instability and proneness to malignancy. Between 80 and 90% of patients with XP have a defect, demonstrable at cell level, of excision of DNA lesions induced by ultraviolet rays, while the remainder have a cellular error of post-replication repair. XP cells are also deficient in repairing DNA damage caused by a variety of chemical mutagens. There are at least five different complementation groups of the first, or classical, type of XP (A to D, etc.) Apparently group C patients, as well as those with defective post-replication repair, do not show the progressive neurological illness found in a proportion of the other patients. AT is heterogeneous clinically and genetically. Clinically it presents with a progressive neurological illness, progressive telangiectases and a developmental disorder of the thymus. AT is characterized by sensitivity to X-rays and AT cells are unable to repair gamma-ray-induced damage to bases in the DNA. It appears that in many cases of the disorder a chromosomally marked cellular clone is found. In BS the main defect, which results in growth retardation, sun-induced lesions of the face and susceptibility to infection, appears to be a slow DNA chain maturation during DNA synthesis. An increase of sister chromatid exchanges is characteristically seen in the chromosomes of cultured BS cells. In FA, in which there is progressive pancytopenia with eventual bone marrow exhaustion and a tendency to haemorrhage and infection, the cellular defect seems to consist of faulty removal of repair of cross-links in the DNA. In this condition, as in BS and AT, various structural chromosome changes are detected in cultured cells. Patients with XP develop skin cancers in early life and often maligant melanomas. In the other three disorders, in which an immune deficiency is often present, leukaemia and related proliferative disorders are a frequent cause of death while other malignancies also occur. There is some evidence that points to an increased risk of malignancy in heterozygotes who carry the FA and AT genes.

Preferential X inactivation in human placenta membranes: is the paternal X inactive in early embryonic development of female mammals?

In placenta membranes of newborn girls carrying electrophoretically distinguishable G6PD alleles, the maternally derived isozyme is expressed preferentially. This phenomenon cannot be explained by allelic differences in enzyme activity or by somatic selection directed against cells with particular G6PD phenotypes. Instead, it may be that in this tissue X inactivation is nonrandom. Preferential expression of the maternal X chromosome, as has been shown in marsupials and in extraembryonic membranes of rodents and now in man, may reflect the state of activity of the X chromosomes in the early stages of female embryonic development.

X-chromosome inactivation during differentiation of female teratocarcinoma stem cells in vitro

Evidence is presented that both X chromosomes are genetically active in clonal cultures of undifferentiated female mouse teratocarcinoma stem cells derived from a spontaneous ovarian tumour. As the cells differentiate in vitro one of the X chromosomes becomes inactivated.

X-chromosome replication in parthenogenic benign ovarian teratomas

Somatic cells from human females undergo X-differentiation, which curtails expression of most, if not all, of the genes on one X-chromosome. According to the Lyon hypothesis, the designation of whick X will be inactive in eutherian females is random. However, in spite of the obvious biologic importance of X chromosome differentiation, little is known about either the mechanism of this process or the role played by fertilization. Benign ovarian teratomas provide a system for assessing the importance of fertilization in X chromosome differentiation. Biochemical and cytologic data indicate that these teratomas are of germ-cell origin. They are comprised entirely of one of the products of the first meiotic division and are thus parthenogens. In the absence of appropriate recombinational events, ovarian teratomas are consistently homozygous at autosomal loci, even when the host is heterozygous. Analysis of X chromosome replication kinetics provides one additional approach for investigating X-differentiation in individual teratoma cells. We utilized BrdU-dye techniques to study terminal replication patterns in ovarian teratomas and in normal fibroblasts and peripheral lymphocytes from the same individuals. The results confirm that human ovarian teratomas possess a single late-replicating X chromosome. Moreover, the pattern of replication in this X is identical to that in normal fibroblasts, but different from that usually observed in peripheral lymphocytes. Thus, if late replication is an accurate gauge of X-inactivation, the data confirm that X-inactivation can occur without fertilization.

Multicellular origin of parathyroid "adenomas"

Most cases of primary hyperparathyroidism are associated with enlargement of a single gland (i.e., an "adenoma") or with chief-cell hyperplasia, but there is controversy about the relative frequency of each of these entities. It has even been postulated that adenomas do not arise spontaneously, bu- result from prolonged hyperplasia in response to unknown stimuli. We studied four parathyroid adenomas from three women with heterozygosity (GdB/GdA) for the X-chromosome-linked enzyme, glucose-6-phosphate dehydrogenase, to determine the number of cells from which the growths arise. Unicellular origin would be compatible with a rare oncogenic event, whereas multicellular origin might be seen with hyperplasia. Both B and A isoenzymes were found in each "adenoma" in proportions similar to those observed in normal tissues, indicating that the lesions have multicellular origin. Thus, parathyroid hyperplasia and adenomas, which at best are difficult to distinguish from one another pathologically, may also be similar biologically.

The selection of somatic cell hybrids between human lymphoma cell lines

Somatic cell hybrids have been selected between three pairs of established human lymphoid cell lines producing pure lines of proliferating hybrid cells: Raji/Namalwa, Raji/Daudi, and Raji/BJAB. The hybrid cell lines have been characterized with respect to isozyme pattern, volume, and karyotype.

Evidence for two active X chromosomes in a human XXY triploid

A human XXY chromatin negative triploid culture has been found to have both the A and B isozymes of glucose-6-phosphate dehydrogenase, as well as an intermediately migrating AB (hybrid) band. This finding indicates that the cells in this culture each possess 2 active X chromosomes. Possible mechanisms producing the abnormalities seen in this disorder are presented and discussed.

Localized Derepression on the Human Inactive X Chromosone in Mouse-Human Cell Hybrids

Evidence for derepression of the gene for hypoxanthine phosphoribosyltransferase (HPRT; IMP: pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) on the human inactive X chromosome was obtained in hybrids of mouse and human cells. The mouse cells lacked HPRT and were also deficient in adenine phosphoribosyltransferase (APRT; AMP: pyrophosphate phosphoribosyltransferase; EC2.4.2.7). The human female fibroblasts were HPRT-deficient as a consequence of a mutation on the active X but contained a normal HPRT gene on the inactive X. The two human X chromosomes were further distinguished by differences in morphology: the inactive X was morphologically normal while the active X included most of the long arm of autosome no. 1 translocated to the distal end of the X long arm. Forty-one hybrid clones were first isolated by selection for the presence of APRT; when these clones were selected for HPRT, six of them yielded derivatives having human HPRT with incidences of about 1 in 10-6 APRT-selected hybrid cells. The HPRT-positive derivatives contained a normal-appearing X chromosome indistinguishable from the inactive X of the parental human fibroblasts. The active X with the translocation was not found in any of the HPRT-positive hybrid cells. Human phosphoglycerokinase (ATP:3-phospho-D-glycerate 1-phosphotransferase. EC 2.7.2.3) and glucose-6-phosphate dehydrogenase (D-glucose 6-phosphate: NADP 1-oxidoreductase, EC 1.1.1.49), which are specified by X-chromosomal loci, were not detected in the hybrids expressing HPRT even though they contained an apparently intact X chromosome. The observations are most simply explained by the infrequent, stable derepression of inactive X chromosome segments that include the HPRT locus but not the phosphoglycerokinase and glucose-6-phosphate dehydrogenase loci.

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.