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

Skewed X-Chromosome Inactivation and Parental Gonadal Mosaicism Are Implicated in X-Linked Recessive Female Hemophilia Patients

Background: Hemophilia A (HA) and B (HB) are X-linked recessive disorders that mainly affect males born from a mother carrier. Females are rarely affected but a number of mechanisms have been suggested in symptomatic females, such as skewed X-chromosome inactivation (XCI), chromosomal rearrangements, and hermaphrodites. Different methodologies are required to elucidate the underlying causes of such diseases in female patients. Methods: Three families with female hemophilia patients, including two HA and one HB, were enrolled for genetic analyses. Cytogenetics, molecular examinations on F8 and F9 genes, XCI assay, and linkage analysis were performed. Results: All three female patients are demonstrated to be heterozygous for an F8, or F9 mutation: one patient is inherited from her unaffected mother and the other two are sporadic cases. All three patients exhibit skewed XCI. The inherited patient is found to be unmethylated in the maternal X chromosome, which increases the potential for the expression of the mutant allele. The two sporadic cases are hypomethylated or unmethylated in the paternal X chromosome, suggesting that paternal gonadal mosaicism may exist in these families. Conclusions: In addition to screening for coagulation function, different genetic analyses are mandatory to explore the nature of mechanisms responsible for the X-linked recessive disorders in female patients as shown in this study. Our results confirm that skewed XCI is responsible for hemophilia in heterozygous female patients. Likewise, our results implicate that parental gonadal mosaicism, followed by skewed XCI, contributes to hemophilia in “sporadic” female patients.

American marsupials chromosomes: Why study them?

Marsupials, one of the three main groups of mammals, are only found in Australia and in the American continent. Studies performed in Australian marsupials have demonstrated the great potential provided by the group for the understanding of basic genetic mechanisms and chromosome evolution in mammals. Genetic studies in American marsupials are relatively scarce and cytogenetic data of most species are restricted to karyotype descriptions, usually without banding patterns. Nevertheless, the first marsupial genome sequenced was that of Monodelphis domestica, a South American species. The knowledge about mammalian genome evolution and function that resulted from studies on M. domestica is in sharp contrast with the lack of genetic data on most American marsupial species. Here, we present an overview of the chromosome studies performed in marsupials with emphasis on the South American species.

Intralocus sexual conflict

Intralocus sexual conflict arises when there are sex-specific optima for a trait that is expressed in both sexes and when the constraint of a shared gene pool prevents males and females from reaching their optima independently. This situation may result in a negative intersexual correlation for fitness. Here I first discuss key differences between intra- and interlocus conflict, the type of sexual conflict that arises in mating interactions between males and females. I then review the experimental evidence for the existence of genomewide sexually antagonistic variation and discuss how intralocus conflict can be resolved. Substantial genomewide sexually antagonistic variation exists in Drosophila melanogaster lab populations. Yet, in the same species, sex-specific gene regulation appears to evolve rapidly, suggesting that the obstacles to the resolution of intralocus conflict are minor. The fact that negative intersexual correlations for fitness are observed even if sexual dimorphism can evolve rapidly suggests that intralocus conflict is highly dynamic. The final part of this review examines the evolutionary consequences of intralocus sexual conflict for the evolution of the sex chromosomes, sexual selection, and sex determination. Intralocus conflict helps to explain many of the peculiar features of the sex chromosomes and has shaped the functional bias and expression biases of sex-linked genes. The genomic distribution of sexually selected genes, in particular, affects sexual selection in various ways. The presence of sexually antagonistic variation can strongly interfere with the good genes' process of sexual selection and erode the genetic benefits of mate choice. Regarding sex determination, this review concentrates on evolutionary transitions between different sex determination mechanisms. Such transitions have occurred frequently in several taxa. Theory and empirical data suggest an important role for intralocus conflict in triggering switches between sex determination systems.

Heterochromatin as a factor affecting X-inactivation in interspecific female vole hybrids (Microtidae, Rodentia)

Female interspecific vole hybrids were examined for the expression of the G6PD and GALA genes on the X chromosomes. When one of the parents was a species with a heterochromatin block on the X, and the other parent was M. arvalis, without a heterochromatin block on the X, preferential expression of the genes of the M. arvalis X was consistently observed. When both parental species had heterochromatin on the X, the parental forms of G6PD and GALA were in about equal proportions in the hybrid females. The results of the cytological identification of the active and inactive X on the metaphase spreads in the hybrid females are in agreement with the biochemical results. It is suggested that the observed phenomenon may be due to a nonrandom inactivation of the X chromosome containing a heterochromatin block in crosses involving M. arvalis and by a random inactivation in those with both parents having heterochromatin blocks on the X chromosomes. These results support our previous suggestion that heterochromatin has an effect on X inactivation in female interspecific vole hybrids.

Differential expression of sex-linked and autosomal germ-cell-specific genes during spermatogenesis in the mouse

We have examined expression during spermatogenesis in the mouse of three Y-linked genes, 11 X-linked genes and 22 autosomal genes, all previously shown to be germ-cell-specific and expressed in premeiotic spermatogonia, plus another 21 germ-cell-specific autosomal genes that initiate expression in meiotic spermatocytes. Our data demonstrate that, like sex-linked housekeeping genes, germ-cell-specific sex-linked genes are subject to meiotic sex-chromosome inactivation (MSCI). Although all the sex-linked genes we investigated underwent MSCI, 14 of the 22 autosomal genes expressed in spermatogonia showed no decrease in expression in meiotic spermatocytes. This along with our observation that an additional 21 germ-cell-specific autosomal genes initiate or significantly up-regulate expression in spermatocytes confirms that MSCI is indeed a sex-chromosome-specific effect. Our results further demonstrate that the chromosome-wide repression imposed by MSCI is limited to meiotic spermatocytes and that postmeiotic expression of sex-linked genes is variable. Thus, 13 of the 14 sex-linked genes we examined showed some degree of postmeiotic reactivation. The extent of postmeiotic reactivation of germ-cell-specific X-linked genes did not correlate with proximity to the X inactivation center or the Xist gene locus. The implications of these findings are discussed with respect to differential gene regulation and the function of MSCI during spermatogenesis, including epigenetic programming of the future paternal genome during spermatogenesis.

Sexual antagonism and X inactivation--the SAXI hypothesis

X inactivation has evolved in the soma of mammalian females so that both sexes have the same ratio of X:autosomal gene expression. The X chromosome in the germ cells of XY males is also precociously inactivated for reasons that remain unclear. Unlike X inactivation in the soma, this germline X inactivation is not restricted to mammals but has evolved independently in several animal phyla. Thus, germline X inactivation might have been the precursor of somatic X inactivation in mammals. We now propose a hypothesis for the evolution of germline X inactivation. The hypothesis predicts a redistribution of late spermatogenic genes from the X chromosome to the autosomes, leading eventually to germline X inactivation as the X chromosome becomes 'demasculinized'. Sexual antagonism could be the mechanism driving this redistribution. Recent expression and genetic studies in mammals, nematodes and Drosophila support this hypothesis, and expression data on taxa that have not evolved germline X inactivation, such as birds and butterflies, should shed further light on it.

X-chromosome inactivation during spermatogenesis is regulated by an Xist/Tsix-independent mechanism in the mouse

Transcriptional inactivation of the single X chromosome occurs in spermatogenic cells during male meiosis in mammals and has been shown to be coincident with expression of the Xist gene in spermatogonia and spermatocytes in mice. However, male mice carrying an ablated Xist gene show normal fertility. Here we examined expression from the Xist locus during spermatogenesis in wild-type mice and detected sense (Xist), but not antisense (Tsix) transcripts. In addition, we examined expression and chromatin conformation of X-linked structural genes in meiotic and postmeiotic spermatogenic cells from wild-type and Xist(-) mice and found no differences associated with the absence of a functional Xist gene. These results, along with the formation of a morphologically normal XY body in primary spermatocytes in Xist(-) mice, indicate that a functional Xist gene is not required for X-chromosome inactivation during spermatogenesis and that this process is therefore regulated by a different mechanism than that which regulates X-chromosome inactivation in female embryonic cells.

Germ cell development in the XXY mouse: evidence that X chromosome reactivation is independent of sexual differentiation

Prior to entry into meiosis, XX germ cells in the fetal ovary undergo X chromosome reactivation. The signal for reactivation is thought to emanate from the genital ridge, but it is unclear whether it is specific to the developing ovary. To determine whether the signals are present in the developing testis as well as the ovary, we examined the expression of X-linked genes in germ cells from XXY male mice. To facilitate this analysis, we generated XXY and XX fetuses carrying X chromosomes that were differentially marked and subject to nonrandom inactivation. This pattern of nonrandom inactivation was maintained in somatic cells but, in XX as well as XXY fetuses, both parental alleles were expressed in germ cell-enriched cell populations. Because testis differentiation is temporally and morphologically normal in the XXY testis and because all germ cells embark upon a male pathway of development, these results provide compelling evidence that X chromosome reactivation in fetal germ cells is independent of the somatic events of sexual differentiation. Proper X chromosome dosage is essential for the normal fertility of male mammals, and abnormalities in germ cell development are apparent in the XXY testis within several days of X reactivation. Studies of exceptional germ cells that survive in the postnatal XXY testis demonstrated that surviving germ cells are exclusively XY and result from rare nondisjunctional events that give rise to clones of XY cells.

Germ cell loss in the XXY male mouse: altered X-chromosome dosage affects prenatal development

Male mammals with two X chromosomes are sterile due to the demise of virtually all germ cells; however, the underlying reasons for the germ cell loss remain unclear. The use of a breeding scheme for the production of XXY male mice has allowed us to experimentally address the question of when and why germ cells die in the XXY testis and whether the defect is due to the presence of an additional X chromosome in the soma, the germ cells themselves, or both. Our studies demonstrate that altered X-chromosome dosage acts to impair germ cell development in the testis at a much earlier stage than suggested by previous studies of XX sex-reversed males or XX/XY chimeras. Specifically, we noted significantly reduced germ cell numbers in the XXY testis during the period of germ cell proliferation in the early stages of testis differentiation. Although the somatic development of the XXY testis is morphologically and temporally normal, our studies indicate that germ cell demise reflects a defect in somatic/germ cell communication, since, in an in vitro system, the proliferative potential of fetal germ cells from XXY males is indistinguishable from that of normal males.

Aetiology of intersexuality in female (XX) pigs, with novel molecular interpretations

This review considers the problem of ovotestis formation in animals of 38,XX chromosome complement. After a clinical description, attention focuses on the condition of the gonads and genital tract. A complete spectrum of gonadal types has been found, ranging from a single ovotestis almost invariably on the right-hand side to both gonads appearing as testicular-like structures, sometimes with a distinct tunica albuginea. The ovotestis or testis-like structure may have descended to an inguinal or scrotal location. Although interstitial cells of Leydig and seminiferous tubules were always abundant in testicular tissue, germ cells were never present. The lumen of the seminiferous tubules was packed with pale-staining, Sertoli-like cells. A bicornuate uterus was characteristic but suppression of the proximal portion of the Müllerian duct always adjoined an ovotestis; a corresponding development of the Wolffian duct featured as a convoluted epididymis. Inhibition of the Fallopian tube was attributed to a local influence of AMH from the Sertoli cells, as was the failure of small Graafian follicles within an ovotestis to respond to injected gonadotrophins. As to the aetiology of an ovotestis, defective colonisation of the genital ridges by primordial germ cells is considered, as is evidence for incorporation of adrenal cells into the embryonic gonad. Molecular probing has failed to reveal the classical sex-determining gene, Sry, and other Y-related DNA sequences such as Zfy and DYZI in almost all the intersex animals examined. Currently favoured as an explanation for ovotestis formation is a mutation in the inhibin gene within granulosa cells of Graafian follicles. Such a mutation would prompt secretion of the closely comparable glycoprotein molecule AMH in these genetic females, with a resultant progressive virilisation of gonadal tissue. The proposed mutation may be carried as an autosomal recessive gene by certain boars. Varying amounts of AMH secretion or differing timescales for the transition from inhibin to AMH could in part explain differing degrees of ovotestis formation. Despite this proposition, interactions between genes that prescribe functional testicular tissue, enhanced rates of gonadal development, and left-right asymmetries between the paired gonads now require systematic study.

Expression of Xist during mouse development suggests a role in the initiation of X chromosome inactivation

The mouse Xist gene maps to the X inactivation center (Xic) region and is expressed exclusively from the inactive X chromosome. It is thus a candidate gene for the Xic. We show that the onset of Xist expression in mouse development precedes X chromosome inactivation and may therefore be a cause rather than merely a consequence of X inactivation. The earliest Xist expression in morulae and blastocysts is imprinted, resulting in specific expression of the paternal Xist allele. Imprinted Xist expression may thus be the cause of nonrandom inactivation of the paternal X in trophectoderm. Strong Xce alleles can act to reduce the effect of imprinted Xist expression in the trophectoderm. The imprint on Xist expression is lost shortly before gastrulation when random X inactivation occurs. Our data support a direct role for Xist in the initiation of X inactivation.

Mammalian sex chromosomes: evolution of organization and function

Comparisons of chromosome size, morphology and gene arrangements between mammals of different species permit us to deduce the genome characteristics of the common ancestor, and to chart the changes that have occurred during the divergence of the two lineages. The more distantly related are the species compared, the more remote the common ancestor whose characteristics can be deduced. This means that, providing there are sufficient similarities to warrant comparison, the more divergent the species compared, the more significant the contribution to our understanding of the organization of an ancestral mammalian genome and the process of mammalian genome evolution. One of the genetic surprises of the last decade was the discovery that, although gross karyotypes of distantly related orders of eutherian mammals (e.g. cat, cow, rabbit, man) have diverged extensively, gene mapping studies reveal the presence of large chromosome segments conserved across at least 60 million years (O'Brien et al. 1988). This finding makes it worthwhile to extend genetic comparisons to the two groups of mammals most distantly related to eutherian mammals--marsupials and monotremes. Here we will review comparisons of the sex chromosomes in these three major groups of extant mammals, and show how they have led us to a new view of the evolution of mammalian sex chromosome organization and function in sex determination and X chromosome inactivation.

The evolution of heteromorphic sex chromosomes

The facts and ideas which have been discussed lead to the following synthesis and model. 1. Heteromorphic sex chromosomes evolved from a pair of homomorphic chromosomes which had an allelic difference at the sex-determining locus. 2. The first step in the evolution of sex-chromosome heteromorphism involved either a conformational or a structural difference between the homologues. A structural difference could have arisen through a rearrangement such as an inversion or a translocation. A conformational difference could have occurred if the sex-determining locus was located in a chromosomal domain which behaved as a single control unit and involved a substantial segment of the chromosome. It is assumed that any conformational difference present in somatic cells would have been maintained in meiotic prophase. 3. Lack of conformational or structural homology between the sex chromosomes led to meiotic pairing failure. Since pairing failure reduced fertility, mechanisms preventing it had a selective advantage. Meiotic inactivation (heterochromatinization) of the differential region of the X chromosome in species with heterogametic males and euchromatinization of the W in species with heterogametic females are such mechanisms, and through them the pairing problems are avoided. 4. Structural and conformational differences between the sex chromosomes in the heterogametic sex reduced recombination. In heterogametic males recombination was reduced still further by the heterochromatinization of the X chromosome, which evolved in response to selection against meiotic pairing failure. 5. Suppression of recombination resulted in an increase in the mutation rate and an increased rate of fixation of deleterious mutations in the recombination-free chromosome regions. Functional degeneration of the genetically isolated regions of the Y and W was the result. In XY males this often led to further meiotic inactivation of the differential region of the X chromosome, and in this way an evolutionary positive-feedback loop may have been established. 6. Structural degeneration (loss of material) followed functional degeneration of Y or W chromosomes either because the functionally degenerate genes had deleterious effects which made their loss a selective advantage, or because shorter chromosomes were selectively neutral and became fixed by chance. 7. The evolutionary routes to sex-chromosome heteromorphism in groups with female heterogamety are more limited than in those with male heterogamety. Oocytes are usually large and long-lived, and are likely to need the products of X- or Z-linked genes. Meiotic inactivation of these chromosomes is therefore unlikely. In the oocytes of ZW females, meiotic pairing failure is avoided through euchromatinization of the W rather than heterochromatinization of the Z chromosome.(ABSTRACT TRUNCATED AT 400 WORDS)

Linear growth in patients with hypophosphatemic vitamin D-resistant rickets: influence of treatment regimen and parental height

The effects of different treatment regimens and the influence of parental height on the statural growth of 40 patients with hereditary vitamin D-resistant hypophosphatemic rickets were investigated. Three treatment regimens, each with oral phosphate, were used: vitamin D (0.5 to 2 mg/day), calcidiol (50 to 200 micrograms/day), and 1 alpha-hydroxyvitamin D3 (1 to 3 micrograms/day). Mean duration of follow-up was 9.5 +/- 5.1 years. The results show that (1) there was no acceleration of growth before puberty for the majority of children treated with vitamin D (12/16) or calcidiol (13/15), whereas 1 alpha-hydroxyvitamin D3 promoted catch-up growth in 10 of 16 patients; (2) height gain during puberty was normal, irrespective of the treatment; (3) most vitamin D-treated male and female subjects and calcidiol-treated male subjects had short adult stature, but the majority (75%) of the 1 alpha-hydroxyvitamin D3-treated groups had normal stature; (4) parental stature had little influence on the adult height of male subjects, but that of affected girls was positively correlated (p less than 0.002) with mid-parental height. These results demonstrate that 1 alpha-hydroxyvitamin D3 is superior to vitamin D or calcidiol for improvement of stature of patients with hypophosphatemic vitamin D-resistant rickets, and indicate the importance of parental height in determining the adult height of affected girls.

X-linked gene expression in metatherian fibroblasts: evidence from the Gpd and Pgk-A loci of the Virginia opossum and the red-necked wallaby

Fibroblasts cultured from ear pinna biopsies of Virginia opossums (Didelphis virginiana) and red-necked wallabies (Macropus rufogriseus) were examined electrophoretically to determine the relative expression levels of the maternally and paternally derived alleles at X-linked, enzyme-coding loci. Only the maternally derived allele was expressed at the Pgk-A locus in fibroblasts of heterozygous D. virginiana (M. rufogriseus not examined), but fibroblasts of both species exhibited evidence of paternal allele expression at the Gpd locus. Furthermore, the heterozygous G6PD phenotypes in both species were skewed in favor of the maternal gene product, as expected if the paternal allele is only partially (incompletely) expressed. For M. rufogriseus this result is contrary to a previous finding which suggested equal expression of both Gpd alleles in cultured fibroblasts of this species. The present results suggest that X-linked genes in metatherian fibroblasts are subject to the same kind of determinate, paternal allele inactivation, incomplete at some loci, described previously for X-linked genes in adult tissues and that the pattern of paternal X-linked gene expression in these cells is independent of the patterns in the tissues from which the fibroblasts are derived.

Characterization of the repressed 5S DNA minichromosomes assembled in vitro with a high-speed supernatant of Xenopus laevis oocytes

We describe an in vitro system, based on the Xenopus laevis oocyte supernatant of Glikin et al. (G. Glikin, I. Ruberti, and A. Worcel, Cell 37:33-41, 1984), that packages DNA into minichromosomes with regularly spaced nucleosomes containing histones H3, H4, H2A, and H2B but no histone H1. The same supernatant also assembles the 5S RNA transcription complex; however, under the conditions that favor chromatin assembly, transcription is inhibited and a phased nucleosome forms over the 5S RNA gene. The minichromosomes that are fully loaded with nucleosomes remain refractory to transcriptional activation by 5S RNA transcription factors. Our data suggest that this repression is caused by a nucleosome covering the 5S RNA gene and that histone H1 is not required for regular nucleosome spacing or for gene repression in this system.

Characterization of the repressed 5S DNA minichromosomes assembled in vitro with a high-speed supernatant of Xenopus laevis oocytes

We describe an in vitro system, based on the Xenopus laevis oocyte supernatant of Glikin et al. (G. Glikin, I. Ruberti, and A. Worcel, Cell 37:33-41, 1984), that packages DNA into minichromosomes with regularly spaced nucleosomes containing histones H3, H4, H2A, and H2B but no histone H1. The same supernatant also assembles the 5S RNA transcription complex; however, under the conditions that favor chromatin assembly, transcription is inhibited and a phased nucleosome forms over the 5S RNA gene. The minichromosomes that are fully loaded with nucleosomes remain refractory to transcriptional activation by 5S RNA transcription factors. Our data suggest that this repression is caused by a nucleosome covering the 5S RNA gene and that histone H1 is not required for regular nucleosome spacing or for gene repression in this system.

The diagnosis and treatment of infants with intersex abnormalities

Management of these children, which begins in the newborn period and often extends throughout adolescence, should be done as a team approach with pediatric surgeons/urologists and pediatric endocrinologists. Careful attention must be paid to the technical aspects of these repair to ensure successful anatomic, functional, and psychological outcome for these often difficult patients. Although this article concentrates on the technical aspects of the surgical repairs of these children, careful consideration is given to the circumstances dictating gender assignment and the factors that affect the choice and timing of operation.

Polymorphism of photopigments in the squirrel monkey: a sixth phenotype

We describe here a trichromatic type of squirrel monkey that resembles Old World monkeys in having two well-separated photopigments in the red-green part of the spectrum; the cones of this phenotype have peak sensitivities close to 430, 536 and 564 nm. The existence of such animals is predicted by a genetic model that postulates three alleles for a single locus on the X-chromosome of the squirrel monkey. The three alleles correspond to three different photopigments in the red-green spectral range. A male monkey, or a homozygous female, will be dichromatic, combining short-wave cones with just one of the cone types in the red-green range. But a female monkey, if heterozygous at the locus, draws any two of the three alleles from the set. X-chromosome inactivation ensures that the two alleles are expressed in different subpopulations of retinal cone, giving the monkey the basis for trichromatic colour vision. This model requires three trichromatic types of female squirrel monkey. The photopigment complements of two types have previously been reported and microspectrophotometric data are now given for the third type. Behaviourally, this third type of trichromat gives precise Rayleigh matches that are intermediate between those of the other two types of trichromat. The polymorphism of photopigments in the squirrel monkey may be maintained by the heterozygous advantage enjoyed by the trichromatic females. This would be an instructive instance of heterozygous advantage because it is a case where X-chromosome inactivation plays a crucial role in segregating the two different gene-products into different cells.

Non-random inactivation of the X-chromosome in interspecific hybrid voles

Expression ofX-linked genes for G6PD and GALA in interspecific hybrids betweenMicrotus arvalis, M. subarvalisandM. kirgisorumvoles was studied. Quantitative predominance of the enzyme activities ofM. arvalisover G6PD activity ofM. subarvalisand the GALA activity ofM. kirgisorumin the female hybrids was shown. The definitive patterns of these enzyme activities was found on day 6·5 of embryonic development. Non-random inactivation ofX-chromosomes derived fromM. subarvalisandM. kirgisorumin the interspecific hybrids withM. arvalisis supposed to be the cause of the phenomenon observed. A hypothesis is proposed that there is a connection between the presence of large heterochromatin regions in theX-chromosomes derived fromM. subarvalisorM. kirgisorumand the preferential inactivation of these in female hybrids withM. arvalis.

What causes the abnormal phenotype in a 49,XXXXY male?

The chromosome replication pattern of a man with 49,XXXXY was analyzed using 3H-thymidine and autoradiography as well as BrdU and acridine orange. The former technique showed a highly irregular replication pattern; the latter revealed one early replicating X chromosome, and the other three more or less asynchronously replicating. Two hypotheses seem to explain best the abnormal phenotype of males with an XXXXY sex chromosome constitution: The number of the always active regions (tip of Xp) and of the possibly always active regions (the Q-dark regions on both sides of the centromere) is increased from one to four. The replication pattern of the late-replicating X chromosomes is highly asynchronous, which might affect the phenotype. The possibility that more than one X chromosome might remain active in some cells, an even more abnormal and obviously deleterious situation, is still open.

Microdissection and microcloning of the mouse X chromosome

A wild mouse (CD) karyotype in which all the chromosomes bar the X, 19, and Y, are fused as metacentrics has been used for the microdissection and microcloning of a specific mouse X chromosome region. Dissection of a proximal region of the X chromosome encompassing the genetic loci Hprt to Tfm and including mdx has yielded 650 clones. A number of the recovered clones containing sizable inserts have been confirmed as X chromosome specific. This X chromosome bank of clones provides a start point for the isolation of the mdx locus. It is now clear that microdissection and microcloning can be applied to all mouse chromosomes, including the X chromosome, yielding premapped banks of clones that will greatly aid in the isolation and characterization of important genetic loci.

Parental source of chromosome imprinting and its relevance for X chromosome inactivation

In imprinting, homologous chromosomes behave differently during development according to their parental origin. Typically, paternally derived chromosomes are preferentially inactivated or eliminated. Examples of such phenomena include inactivation of the mammalian X chromosome, inactivation or elimination of one haploid chromosome set in male coccids, and elimination of paternal X chromosomes in the fly Sciara. It has generally been thought that the paternal chromosomes bear an imprint leading to their inactivation or elimination. However, alteration of the parental origin of chromosomes, as in the study of parthenogenotes in mammals and coccids, shows that passage of chromosomes through a male germ cell or fertilization is not essential for inactivation or elimination. It appears that neither chromosome set is programmed to resist or undergo inactivation. Instead the two sets differ in relative sensitivity, and the question is whether the maternal set have an imprint for resistance, or the paternal set one for susceptibility. Very early in development of mammals both X chromosomes are active. This makes it simpler to envisage the maternal X bearing an imprint for resistance to inactivation, which persists through the early developmental period. Similar considerations also apply in coccids and Sciara. Thus, imprinting should be regarded as a phenomenon conferred on the maternal chromosomes in the oocyte. This permits simpler models for the mechanism of X-inactivation, and weakens the case for evolution of X-inactivation from an earlier form of inactivation during male gametogenesis. One may speculate whether imprinting affects timing of gene action in development.

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.

Characterization of a cloned repetitive DNA sequence concentrated on the human X chromosome

A tandemly repeated DNA sequence organized predominantly, if not entirely, in a specific manner on the human X chromosome has been cloned in pBR322 and characterized. The sequence was detected as a 2-kilobase band in ethidium bromide-stained agarose gels of BamHI-digested total human nuclear DNA. Although in situ hybridization of the cloned sequence to human metaphase chromosomes showed a single major site of hybridization at the centromere region of the X chromosome and minor sites of hybridization at several autosomal centromeres, Southern blot analysis of restricted total human DNA indicated that the cloned probe is related to other repeated DNAs, particularly the human alphoid DNAs. Restriction enzyme analysis of the cloned fragment revealed an internal repeat structure based upon multiples of 170 base pairs, confirming this relatedness. All available data, however, suggest that the 2-kilobase spacing of BamHI sites within the repeat may be specific to the X chromosome.

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.

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.

Activity of steroid sulfatase in fibroblasts with numerical and structural X chromosome aberrations

In cultured fibroblasts of patients with numerical and structural X chromosome aberrations the activity of steroid sulfatase (STS) is correlated with the number of functional STS gene copies. While normally, this X-linked gene is not inactivated, our data suggest that it may be subject to inactivation when carried on a structurally altered X-chromosome. Similar inactivation patterns have been reported earlier for the Xg locus which, like STS, is located on the distal portion of Xp.

Assignment by deletion mapping of the steroid sulfatase X-linked ichthyosis locus to Xp223

A male child and his mother who are nullisomic and monosomic, respectively, for the distal portion of Xp because of an unbalanced X-Y translocation were tested for steroid sulfatase activity after clinical examination had yielded evidence for ichthyosis in the boy. Deficiency of steroid sulfatase was found in the male patient, while in his mother enzyme levels were in the heterozygous range. These results, based on cytogenetic evidence obtained with an elongation technique, indicate that the STS locus is at Xp 223.

X chromosome constitution and the human female phenotype

The correlations of abnormal X chromosome constitutions and the resulting phenotypes in the human female are reviewed. The following hypotheses put forward to explain these correlations are discussed in detail: (1) The damage is done before X inactivation; (2) An effect is exerted between reactivation of the X chromosome(s) and meiosis in oocytes; (3) A recessive gene(s) in hemizygous condition might be expressed in the cases in which the same X is active in all cells; (4) A change in the number of presumed active regions on the inactive X chromosomes might have an effect; (5) A position effect, in that the region Xq13-q27 has to be intact in both X chromosomes to allow normal development, may be responsible; (6) An effect during the period when cells with different inactivation patterns compete is a probability; (7) The original X inactivation may be neither regular nor random. The conclusion reached is that the phenotypic effects of a specific X chromosome aberration may be simultaneously exerted through different pathways (Tables 1 and 2). Hypotheses (2), (4), (5), and (6) are considered probable. Hypothesis (3) has been discarded, and there is very little evidence for hypotheses (1) and (7).

A comparison of genetic variability at X-linked and autosomal loci in kangaroos, man and Drosophila

This paper tests the hypothesis that haplodiploidy orXlinkage leads to less genetic variability. Although haplodiploid organisms exhibit a low level of genetic variability the wide variation existing between different diploid organisms implies that factors other than the genetical system could also be responsible. In order to test the hypothesis critically it is necessary to compare the level of genetic variability betweenX-linked and autosomal genes within a closely related group of organisms. For kangaroos, the ascertainment bias forX-linked loci has been removed by assuming the correctness of Ohno's law of conservation of the mammalianX, i.e. that genes found to beX-linked in man can be assumed to beX-linked in kangaroos. For Man andDrosophila, it has been assumed that the percentage of the karyotype which isXchromosome can be used as the expectation for the percentage ofX-linked polymorphisms. No difference between the two classes of loci is evident in kangaroos and man for percentage polymorphism. The data however have confidence limits which would allow autosomal loci to have three times greater percentage polymorphism. InDrosophilathe published data of Prakash show that autosomal loci are polymorphic about twice as frequently as are theirX-linked counterparts. Thus theremaybe a modest reduction in percentage polymorphism as a result ofX-linkage (i.e. haplodiploidy). No reduction in the number of alleles per locus or average heterozygosity at those loci which are polymorphic is evident in kangaroos, man, orDrosophila. More data on moreX-linked enzymes are necessary to establish firmly that there is a real reduction in percentage polymorphism and to estimate its extent. The kangaroo data are incompatible with the hypothesis that a large fraction of the variability is maintained by simple overdominance since overdominance is very unlikely in the quasi-haploid genetical system which results from the paternalXinactivation mode of dosage compensation used by kangaroos. This is the first report on level of enzymic variability in marsupials. 17% of autosomal loci and 18% ofX-linked loci are polymorphic, average heterozygosity is 4% for autosomal and 4% forX-linked loci and number of alleles per locus is 1·25 for autosomal and 1·21 forX-linked loci. These figures are somewhat lower than for eutherian mammals.

H-Y antigen mosaicism in the gonad of a 46,XX true hermaphrodite

To clarify the mechanism for the coexistence of ovarian and testicular tissue in the 46,XX true hermaphrodite, we studied a 20-year-old phenotypic male with gynecomastia and elevated serum concentrations of estradiol in whom an ovotestis was discovered upon scrotal exploration. Y chromosomal material could not be detected by fluorescent Y-body analysis or Giemsa-banding technics in cells cultured from peripheral blood, breast or forearm skin or the ovarian or testicular portions of the ovotestis. However, serologic testing, using the sperm cytotoxicity assay, revealed that cells cultured from the testicular portion of the ovotestis were H-Y antigen positive whereas cells cultured from the ovarian portion were H-Y antigen negative. These observations indicate that the ovotestis arises from an H-Y+/H-Y- mosaic primordium.