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.     

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

Abstract. 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.     

The late-replicating X chromosome in digynous mouse triploid embryos

Using BrdU-labeling and acridine orange staining, the behavior of X-chromosome replication was studied in 28 XXX and 19 XXY digynous mouse triploids. In some of these the paternal and maternal X chromosome could by cytologically distinguished. Such embryos were obtained by mating chromosomally normal females with males carrying Cattanach's X chromosome which contains an autosomal insertion that substantially increases the length of this chromosome. In the XXX triploids there were two distinct cell lines, one with two late-replicating X chromosomes, and the other with only one late-replicating X. The XXY triploids were also composed of two cell populations, one with a single late-replicating X and the other with no late replicating X chromosome. Assuming that the late-replicating X is genetically inactive, in both XXX and XXY triploids, cells from the embryonic region tended to have only one active X chromosome, whereas those from the extra-embryonic membranes tended to have two active X chromosomes. The single active X chromosome was either paternal or maternal in origin, but two active X chromosomes were overwhelmingly maternal in origin, suggesting paternal X-inactivation in extra-embryonic tissues.     

Chromosome elimination in sciarid flies

The programmed elimination of part of the genome through chromosome loss or chromatin diminution constitutes an exceptional biological process found to be present in several diverse groups of organisms. The occurrence of this phenomenon during early embryogenesis is generally correlated to somatic versus germ-line differentiation. A most outstanding example of chromosome elimination and genomic imprinting is found in sciarid flies, where whole chromosomes of exclusive parental origin are selectively eliminated at different developmental stages. Three types of tissue-specific chromosome elimination events occur in sciarids. During early cleavages, one or two X paternal chromosomes is/are discarded from somatic cells of embryos which then develop as females or males respectively. Thus, the sex of the embryo is determined by the number of eliminated paternal X chromosomes. In germ cells, instead, a single paternal X chromosome is eliminated in embryos of both sexes. In addition, while female meiosis is orthodox, male meiosis is highly unusual as the whole paternal chromosome set is discarded from spermatocytes. As a consequence, only maternally derived chromosomes are included in the functional sperm. This paper reviews current cytological and molecular knowledge on the tissue-specific cell mechanisms evolved to achieve chromosome elimination in sciarids.     

Molecular and cytological characterization of repetitive DNA sequences from the centromeric heterochromatin of <Emphasis Type="Italic">Sciara coprophila</Emphasis>

Sciara coprophila (Diptera, Nematocera) constitutes a classic model to analyze unusual chromosome behavior such as the somatic elimination of paternal X chromosomes, the elimination of the whole paternal, plus non-disjunction of the maternal X chromosome at male meiosis. The molecular organization of the heterochromatin in S. coprophila is mostly unknown except for the ribosomal DNA located in the X chromosome pericentromeric heterochromatin. The characterization of the centromeric regions, thus, is an essential and required step for the establishment of S. coprophila as a model system to study fundamental mechanisms of chromosome segregation. To accomplish such a study, heterochromatic sections of the X chromosome centromeric region from salivary glands polytene chromosomes were microdissected and microcloned. Here, we report the identification and characterization of two tandem repeated DNA sequences from the pericentromeric region of the X chromosome, a pericentromeric RTE element and an AT-rich centromeric satellite. These sequences will be important tools for the cloning of S. coprophila centromeric heterochromatin using libraries of large genomic clones.     

Heterogeneous X inactivation in trophoblastic cells of human full-term female placentas

In female mammalian cells, one of the two X chromosomes is inactivated to compensate for gene-dose effects, which would be otherwise doubled compared with that in male cells. In somatic lineages in mice, the inactive X chromosome can be of either paternal or maternal origin, whereas the paternal X chromosome is specifically inactivated in placental tissue. In human somatic cells, X inactivation is mainly random, but both random and preferential paternal X inactivation have been reported in placental tissue. To shed more light on this issue, we used PCR to study the methylation status of the polymorphic androgen-receptor gene in full-term human female placentas. The sites investigated are specifically methylated on the inactive X chromosome. No methylation was found in microdissected stromal tissue, whether from placenta or umbilical cord. Of nine placentas for which two closely apposed samples were studied, X inactivation was preferentially maternal in three, was preferentially paternal in one, and was heterogeneous in the remaining five. Detailed investigation of two additional placentas demonstrated regions with balanced (1:1 ratio) preferentially maternal and preferentially paternal X inactivation. No differences in ratio were observed in samples microdissected to separate trophoblast and stromal tissues. We conclude that methylation of the androgen receptor in human full-term placenta is specific for trophoblastic cells and that the X chromosome can be of either paternal or maternal origin.     

Analysis of two 47,XXX males reveals X-Y interchange and maternal or paternal nondisjunction

Summary Two cases of 47,XXX males were studied, one of which has been published previously (Bigozzi et al. 1980). Analysis of X-linked restriction fragment length polymorphisms revealed that in this case, one X chromosome was of paternal and two were of maternal origin, whereas in the other case, two X chromosomes were of paternal and one of maternal origin. Southern blot analysis with Y-specific DNA probes demonstrated the presence of Y short arm sequences in both XXX males. In one case, the results obtained pointed to a paracentric inversion on Yp of the patient's father. In situ hybridization indicated that the Y-specific DNA sequences were localized on Xp22.3 in one of the three X chromosomes in both cases. The presence of Y DNA had no effect on random X inactivation. It is concluded that both XXX males originate from aberrant X-Y interchange during paternal meiosis, with coincident nondisjunction of the X chromosome during maternal meiosis in case 1, and during paternal meiosis II in case 2.     

Coincident maternal meiotic nondisjunction of chromosomes X and 21 without evidence of autosomal aysnapsis

Summary A family in which the proband showed phenotypic signs of both the Turner and Down syndromes was studied cytogenetically and with restriction fragment length polymorphisms. The proband's karyotype was 46,X,+21, showing double aneuploidy without any signs of mosaicism. The single X and one chromosome 21 were of paternal origin while two chromosomes 21 were of maternal origin. The nondisjunction of chromosome 21 took place in maternal meiosis II. If it is assumed that the absence of mosaicism renders postzygotic mitotic loss of the X chromosome unlikely, then the X chromosome would have been lost in maternal meiosis I or II. Recombination had occurred between the nondisjoined chromosomes 21. We conclude that double nondisjunction took place in one parent and that asynapsis was not a prerequisite for the autosomal nondisjunction.     

A New Mutant Controlling Mitotic Chromosome Disjunction in DROSOPHILA MELANOGASTER

A new mutant, mit (mitotic loss inducer), is described. The mutant is recessive and maternal in action, producing gynandromorphs and haplo-4 mosaics among the progeny of homozygous mit females. Mosaic loss of maternal or paternal chromosomes can occur. The probabilities of either maternal or paternal X chromosome loss are equal. mit has been mapped to approximately 57 on the standard X chromosome map.-Using gyandromorphs generated by mit, a morphogenetic fate map, placing the origins of 40 cuticular structures on the blastoderm surface, has been constructed. This fate map is consistent with embryological data and with the two other fate maps generated in different ways.     

Parental origin of the extra chromosomes in polysomy X

Five polymorphic index markers were analyzed by polymerase chain reaction (PCR) to ascertain the parental origin of the extra X chromosomes in seven polysomic cases (one 49,XXXXX, three 49,XXXXY, two 48,XXXY, and one 48, XXYY). All four X chromosomes in 49, X polysomies were maternal in origin and the extra X chromosomes in 48 X polysomies were paternal. In each case the multiple X chromosomes were contributed by a single parent. Taken together with previously reported cases, these data support a single mechanism of sequential nondisjunction during either maternal or paternal gametogenesis as the cause of higher order sex chromosome polysomy.     

Sex chromosome elimination,X chromosome inactivation and reactivation in the southern brown bandicoot Isoodon obesulus (Marsupialia: Peramelidae)

Cytogenetic studies have shown that bandicoots (family Peramelidae) eliminate one X chromosome in females and the Y chromosome in males from some somatic tissues at different stages during development. The discovery of a polymorphism for X-linked phosphoglycerate kinase (PGK-1) in a population of Isoodon obesulus from Mount Gambier, South Australia, has allowed us to answer a number of long standing questions relating to the parental source of the eliminated X chromosome, X chromosome inactivation and reactivation in somatic and germ cells of female bandicoots. We have found no evidence of paternal PGK-1 allele expression in a wide range of somatic tissues and cell types from known female heterozygotes. We conclude that paternal X chromosome inactivation occurs in bandicoots as in other marsupial groups and that it is the paternally derived X chromosome that is eliminated from some cell types of females. The absence of PGK-1 paternal activity in somatic cells allowed us to examine the state of X chromosome activity in germ cells. Electrophoresis of germ cells from different aged pouch young heterozygotes showed only maternal allele expression in oogonia whereas an additional paternally derived band was observed in pre-dictyate oocytes. We conclude that reactivation of the inactive X chromosome occurs around the onset of meiosis in female bandicoots. As in other mammals, late replication is a common feature of the Y chromosome in male and the inactive X chromosome in female bandicoots. The basis of sex chromosome loss is still not known; however later timing of DNA synthesis is involved. Our finding that the paternally derived X chromosome is eliminated in females suggests that late DNA replication may provide the imprint for paternal X inactivation and the elimination of sex chromosomes in bandicoots.     

The disparate maternal aunt-uncle ratio in male transsexuals: an explanation invoking genomic imprinting

A significant skewing in the sex ratio in favour of females has been reported for the families of homosexual men such that there are fewer maternal uncles than aunts. This finding is repeated for a large series of transsexual families in this study. Four hundred and seventeen male-to-female transsexuals and 96 female-to-male transsexuals were assessed. Male-to-female transsexuals have a significant excess of maternal aunts vs. uncles. No differences from the expected parity were found for female-to-male transsexuals or on the paternal side. A posited explanation for these findings invokes X inactivation and genes on the X chromosome that escape inactivation but may be imprinted. Our hypothesis incorporates the known familial traits in the families of homosexuals and transsexuals by way of retention of the grand parental epigenotype on the X chromosome. Generation one would be characterized by a failure to erase the paternal imprints on the paternal X chromosome. Daughters of this second generation would produce sons that are XpY and XmY. Since XpY expresses Xist, the X chromosome is silenced and half of the sons are lost at the earliest stages of pregnancy because of the normal requirement for paternal X expression in extra-embryonic tissues. Females survive by virtue of inheriting two X chromosomes, and therefore the possibility of X chromosome counting and choice during embryonic development. In generation three, sons inheriting the paternal X after its second passage through the female germline survive, but half would inherit the feminizing Xp imprinted genes. These genes could pre-dispose the sons to feminization and subsequent development of either homosexuality or transsexualism.     

Concomitant Turner syndrome and hemophilia A in a female with an idic(X)(p11) heterozygous at locus DXS52.

A 46,X,idic(X)(p11) karyotype was found in a female affected by Turner syndrome and sporadic moderate hemophilia A. Restriction fragment length polymorphism analysis of the patients's DNA demonstrated that the idic(X) contained alleles from both maternal X chromosomes. Since the idic(X) appeared to be always inactivated, a de novo mutation of factor VIII in the normal paternal X chromosome is probably responsible for the patient's coagulation disorder.     

Paternal chromosome incorporation into the zygote nucleus is controlled by maternal haploid in Drosophila

maternal haploid (mh) is a strict maternal effect mutation that causes the production of haploid gynogenetic embryos (eggs are fertilized but only maternal chromosomes participate in development). We conducted a cytological analysis of fertilization and early development in mh eggs to elucidate the mechanism of paternal chromosome elimination. In mh eggs, as in wild-type eggs, male and female pronuclei migrate and appose, the first mitotic spindle forms, and both parental sets of chromosomes congress on the metaphase plate. In contrast to control eggs, mh paternal sister chromatids fail to separate in anaphase of the first division. As a consequence the paternal chromatin stretches and forms a bridge in telophase. During the first three embryonic divisions, damaged paternal chromosomes are progressively eliminated from the spindles that organize around maternal chromosomes. A majority of mh embryos do not survive the deleterious presence of aneuploid nuclei and rapidly arrest their development. The rest of mh embryos develop as haploid gynogenetic embryos and die before hatching. The mh phenotype is highly reminiscent of the early developmental defects observed in eggs fertilized by ms(3)K81 mutant males and in eggs produced in incompatible crosses of Drosophila harboring the endosymbiont bacteria Wolbachia.     

Rare etiology of autosomal recessive disease in a child with noncarrier parents

A child with maple syrup urine disease type 2 (MSUD2) was found to be homozygous for a 10-bp MSUD2-gene deletion on chromosome 1. Both purported parents were tested, and neither carries the gene deletion. Polymorphic simple-sequence repeat analyses at 15 loci on chromosome 1 and at 16 loci on other chromosomes confirmed parentage and revealed that a de novo mutation prior to maternal meiosis I, followed by nondisjunction in maternal meiosis II, resulted in an oocyte with two copies of the de novo mutant allele. Fertilization by a sperm that did not carry a paternal chromosome 1 or subsequent mitotic loss of the paternal chromosome 1 resulted in the propositus inheriting two mutant MSUD2 alleles on two maternal number 1 chromosomes.     

The evolution of unusual chromosomal systems in sciarid flies: intragenomic conflict and the sex ratio

A model is presented for the evolution of the sciarid chromosomal system. In this model, a driving X chromosome caused female-biased sex ratios. The drive was exploited by maternal autosomes that segregated with the X at spermatogenesis. Genes in mothers converted some of their XX daughters into sons by eliminating a paternal X from the embryonic soma. L chromosomes were derived from X chromosomes and favored male-biased sex ratios. An X' chromosome arose that suppressed the effects of L chromosomes. The 1:1 sex ratio is a stalemate between the X' and X'X mothers causing all-female broods and the L chromosomes in XX mothers causing all-male broods. Any element (such as an L chromosome) that is preferentially transmitted through one sex will be selected to bias the sex ratio towards this sex.     

The evolution of unusual chromosomal systems in coccoids: extraordinary sex ratios revisited

Coccoids (scale insects) exhibit a wide variety of chromosomal systems. In many species, paternal chromosomes are eliminated from the male germline such that all of a male's sperm transmit an identical set of maternal chromosomes. In such species, an offspring's sex is determined by whether or not paternal chromosomes are inactivated in the egg's cytoplasm after fertilization. This paper presents a model of the evolution of paternal genome loss in coccoids from an ancestral system of XX-XO sex determination. The model is based on Hamilton's (1967) theory that different genetic elements within the genome have different unbeatable sex ratios. In this model (1) meiotic drive by the X chromosome in XO males causes female-biased sex ratios; (2) the maternal set of autosomes in males evolves effective sex linkage to exploit X-drive; and (3) genes expressed in mothers are selected to convert some of their XX daughters into sons. A similar model may explain the evolution of haplodiploidy.     

Determining the origin of human X isochromosomes by use of DNA sequence polymorphisms and detection of an apparent i(Xq) with Xp sequences

Summary The parental origin of five X isochromosomes were determined using 11 DnA markers. The isochromosome was derived from a maternal X chromosome in three cases and from a paternal X chromosome in two. Unexpected heterozygosity was detected for the proximal Xp region in one individual in whom the i(Xq) chromosome was paternally derived. This was confirmed by in situ hybridisation. A mode of formation of isochromosomes by breakage and reunion between the sister chromatids of the arms of an X chromosome is proposed to account for this. Sister chromatid breakage and reunion can be considered as a significant mechanism for the origin of i(Xq) chromosomes.     

Sex Determination in Polyploids of Caenorhabditis Elegans

In Caenorhabditis elegans triploid animals with two X chromosomes (symbolized 3A;2X) are males. However, these triploid males can be feminized by making them mutant for recessive dosage compensation mutations, by adding X chromosome duplications or by microinjecting particular DNA sequences termed feminizing elements. None of these treatments affects diploid males. This study explores several aspects of these treatments in polyploids. The dosage compensation mutants exhibit a strong maternal effect, such that reduction of any of the dosage compensation gene functions in the mother leads to sex reversal of 3A;2X animals. Likewise, all X chromosome duplications tested cause both sex reversal and intersexual development of many 3A;2X animals. Microinjected feminizing element DNA does not cause extensive sex reversal, but does result in intersexual development in 3A;2X animals. Neither X chromosome duplications nor microinjected feminizing elements show the extreme maternal effect of the dosage compensation mutants, although there is indirect evidence for a maternal effect of the feminizing elements. In particular, very little feminizing element DNA needs to be microinjected in order to feminize triploid males, far less than what is needed for stable inheritance, implying that feminizing elements can work within the mother's gonad. However, even very high concentrations of microinjected feminizing elements do not affect sex determination in diploid males, suggesting that they are not part of the numerator of the X/A ratio. In addition, no pair of X chromosome duplications feminizes diploid males, suggesting that none of these duplications contains a numerator of the X/A ratio. Instead, I infer that an X-linked locus, as yet undefined, must be present in two copies for hermaphrodite development to ensue or that the two X chromosomes might interact.