Allelic expression in intergeneric fox hybrids (Alopex lagopus × Vulpes vulpes). III. Regulation of the expression of the parental alleles at the Gpd locus linked to the X chromosome

The electrophoretic pattern of glucose-6-phosphate dehydrogenase (G6PD) was studied in 60 intergeneric fox hybrids (Alopex lagopus × Vulpes vulpes), 33 females and 27 males. It is shown that the structural gene for G6PD, designated Gpd, is located on the X chromosome in both Arctic and silver foxes. Analysis of G6PD patterns in the erythrocytes of hybrid females demonstrated that the phenotypic expression of parental alleles at the Gpd locus varied considerably: from 1:1 to the hemizygous manifestation of an allele of either the Artic or the silver fox. The expression of the parental allels at this locus is different in the various tissues of single female hybrids. It is suggested that the variable quantitative expression of the alleles at the Gpd locus in hybrid females is related to the presence of two cell populations having in an active state either the X chromosome of the Arctic fox or that of the silver fox. It is also proposed that the size of the two cell populations is largely affected by the different relationships between cells having different activated X-chromosomes among initiator (stem) cells from which various definitive organs and tissues develop. The number of initiator cells for erythroid tissue has been calculated to be five or six.     

X-linked gene expression in metatherian fibroblasts: Evidence from theGpd andPgk-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 thePgk-A locus in fibroblasts of heterozygousD. virginiana (M. rufogriseus not examined), but fibroblasts of both species exhibited evidence of paternal allele expression a t theGpd 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. ForM. rufogriseus this result is contrary to a previous finding which suggested equal expression of bothGpd 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.The work was supported in part by grants from the National Institutes of Health (Biomedical Research Support Grant RR-05519) and the National Science Foundation (DCB 8516949).     

Expression of size-polymorphic androgen receptor (AR) gene in ovarian endometriosis according to the number of cytosine, adenine, and guanine (CAG) repeats in AR alleles.

An increase in androgen receptor (AR) caused by estrogen is recognized as one of the biological phenomena related to estrogen-induced growth in uterine endometrium. The A/B region of AR gene in X chromosome involves the cytosine, adenine, and guanine (CAG) repeats. Random X chromosome inactivation with AR alleles in individual cells occurs in females. Therefore, approximately either paternal or maternal single dominant polymorphic AR mRNA must be expressed in neoplastic tissue originated from monoclone. This prompted us to determine deviated number of CAG repeats in AR mRNA to understand clonality in ovarian endometriosis. In all cases of heterozygous AR alleles, although paternal and maternal AR mRNAs from normal eutopic uterine endometrium were consistently expressed as AR alleles, either paternal or maternal single dominant AR mRNA expression was found in an individual ovarian endometrioma. Therefore, an individual ovarian endometrioma might be formed from an independent monoclonal ovarian endometriotic endometrial cell after inactivation of either AR allele in X chromosome.     

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.     

Developmental progression of Gpd expression from the inactive X chromosome of the virginia opossum

Metatherian (marsupial) mammals possess a non-random form of X-chromosome inactivation in which the paternally-derived X is always the one inactivated. To examine the progression of X-linked gene expression during metatherian development, we compared relative levels of the maternally and paternally encoded Gpd gene products in heterozygous female Virginia opossums (Didelphis virginiana) across a moior portion of the developmental period. Panels of tissues obtained from fetuses, newborns, and pouch young were examined via polyacrylamide gel electrophoresis of the G6PD protein. As in adults, G6PD phenotypes in these developmental stages were highly skewed in favor of the maternal allele product, but in some tissues there was a marked increase in paternal allele expression with advancing developmental age. However, even by 42 days of post-partum development, expression of the paternal Gpd allele had not attained the adult, tissue-specific activity pattern. Our findings indicate remarkable developmental changes in the activity of the paternal allele in several tissues/organs continuing well into mid pouch-life stages and beyond. Specifically we found that 1) a substantially repressed paternal Gpdgene is present in the cells of female stage 29 fetuses and later developmental stages, 2) the activity state of the paternal Gpd gene is not fixed during early embryonic development in this species, 3) maior changes in paternal Gpd expression occur in advanced developmental stages and comprise a maturation of the gene expression pattern during ontogeny, and 4) alterations of paternal Gpd allele activity during development occur in a tissue-specific manner. © 1995 Wiley-Liss, Inc.     

Biochemical evidence of haplodiploidy in the whiteflyBemisia tabaci

Polymorphic esterase and acetylcholinesterase alleles in the whiteflyBemisia tabaci were studied using electrophoretic and colorimetric assays. The segregation of these alleles between parental and F₁ generations provided unequivocal evidence of haplodiploidy in this pest species. Unmated females, heterozygous at a polymorphic locus, produced a 1:1 ratio of haploid males expressing either of the maternal alleles. Although male offspring were produced by both virgin and mated females, the segregation of alleles showed they were always haploid (hemizygous) for the marker enzymes. Females only arose from fertilized eggs and invariably expressed paternal and maternal alleles.     

Survival of XO mouse fetuses: effect of parental origin of the X chromosome or uterine environment?

Using a recombinant product from the structurally abnormal Y chromosome, Y*, female mice with a single X of either maternal or paternal origin were generated. The two types of females were produced on the same genetic background and differ only in the origin of the X chromosome. Hence it has been possible to assess the effect of parental origin of the X on survival of females with a single X chromosome. A highly significant prenatal loss of females with a single X of paternal origin, but no comparable loss of females with a single X of maternal origin was observed. The reduced viability of females with a paternally derived X could be mediated by the parental origin of the X (i.e. X chromosome imprinting) or alternatively, since the mothers of females with a single paternally derived X have only a single X chromosome, the effect could be mediated by the genotype of the mother (i.e. maternal uterine effect).     

Preferential expression of the maternally inherited X-linked phosphoglycerate kinase allele in human erythrocytes

Summary The stability of allelic gene expression of X-linked phosphoglycerate kinase was studied in seven carriers of a rare genetic variant named PGK München. The enzymatic activities in erythrocytes of five heterozygous females and three hemizygous males were determined repeatedly over a period of 10 years (1975–1984) and shown to remain constant. As the phosphoglycerate kinase activity is lower in cells expressing the PGK München allele, the ratio of the two cell types in all heterozygous females of the PGK München kindred could be calculated from the PGK activity and from the known allozyme activities in erythrocytes of homozygous wild type or hemizygous PGK München carriers. Since the maternal or paternal origin of both alleles is known from the pedigree, the quantitative expression of the maternally derived allozyme in heterozygous women could be determined. In heterozygous carriers the cell pool expressing the maternally inherited allele was significantly increased, independently, of the PGK allele linked to the maternal X chromosome (P<0.001). Our data show that inactivation of one of the two X chromosomes in human female erythropoietic stem cell precursors may be non-random, at least in the kindred and cell populations described here. The results are discussed in the context of random X chromosome inactivation (Lyon hypothesis).Dedicated to J.S., the senior of the family studied, on the occasion of her 80th birthday     

Gene expression analysis of the ovary of hybrid females of <Emphasis Type="Italic">Xenopus laevis</Emphasis> and <Emphasis Type="Italic">X. muelleri</Emphasis>

Background  

Interspecific hybrids of frogs of the genus Xenopus result in sterile hybrid males and fertile hybrid females. Previous work has demonstrated a dramatic asymmetrical pattern of misexpression in hybrid males compared to the two parental species with relatively few genes misexpressed in comparisons of hybrids and the maternal species (X. laevis) and dramatically more genes misexpressed in hybrids compared to the paternal species (X. muelleri). In this work, we examine the gene expression pattern in hybrid females of X. laevis × X. muelleri to determine if this asymmetrical pattern of expression also occurs in hybrid females.     

The evolution of X-linked genomic imprinting

We develop a quantitative genetic model to investigate the evolution of X-imprinting. The model compares two forces that select for X-imprinting: genomic conflict caused by polygamy and sex-specific selection. Genomic conflict can only explain small reductions in maternal X gene expression and cannot explain silencing of the maternal X. In contrast, sex-specific selection can cause extreme differences in gene expression, in either direction (lowered maternal or paternal gene expression), even to the point of gene silencing of either the maternal or paternal copy. These conclusions assume that the Y chromosome lacks genetic activity. The presence of an active Y homologue makes imprinting resemble the autosomal pattern, with active paternal alleles (X- and Y-linked) and silenced maternal alleles. This outcome is likely to be restricted as Y-linked alleles are subject to the accumulation of deleterious mutations. Experimental evidence concerning X-imprinting in mouse and human is interpreted in the light of these predictions and is shown to be far more easily explained by sex-specific selection.     

X-chromosome inactivation in monkey embryos and pluripotent stem cells

Inactivation of one X chromosome in female mammals (XX) compensates for the reduced dosage of X-linked gene expression in males (XY). However, the inner cell mass (ICM) of mouse preimplantation blastocysts and their in vitro counterparts, pluripotent embryonic stem cells (ESCs), initially maintain two active X chromosomes (XaXa). Random X chromosome inactivation (XCI) takes place in the ICM lineage after implantation or upon differentiation of ESCs, resulting in mosaic tissues composed of two cell types carrying either maternal or paternal active X chromosomes. While the status of XCI in human embryos and ICMs remains unknown, majority of human female ESCs show non-random XCI. We demonstrate here that rhesus monkey ESCs also display monoallelic expression and methylation of X-linked genes in agreement with non-random XCI. However, XIST and other X-linked genes were expressed from both chromosomes in isolated female monkey ICMs indicating that ex vivo pluripotent cells retain XaXa. Intriguingly, the trophectoderm (TE) in preimplantation monkey blastocysts also expressed X-linked genes from both alleles suggesting that, unlike the mouse, primate TE lineage does not support imprinted paternal XCI. Our results provide insights into the species-specific nature of XCI in the primate system and reveal fundamental epigenetic differences between in vitro and ex vivo primate pluripotent cells.     

Imprinting of the MEDEA polycomb gene in the Arabidopsis endosperm.

In flowering plants, two cells are fertilized in the haploid female gametophyte. Egg and sperm nuclei fuse to form the embryo. A second sperm nucleus fuses with the central cell nucleus that replicates to generate the endosperm, which is a tissue that supports embryo development. MEDEA (MEA) encodes an Arabidopsis SET domain Polycomb protein. Inheritance of a maternal loss-of-function mea allele results in embryo abortion and prolonged endosperm production, irrespective of the genotype of the paternal allele. Thus, only the maternal wild-type MEA allele is required for proper embryo and endosperm development. To understand the molecular mechanism responsible for the parent-of-origin effects of mea mutations on seed development, we compared the expression of maternal and paternal MEA alleles in the progeny of crosses between two Arabidopsis ecotypes. Only the maternal MEA mRNA was detected in the endosperm from seeds at the torpedo stage and later. By contrast, expression of both maternal and paternal MEA alleles was observed in the embryo from seeds at the torpedo stage and later, in seedling, leaf, stem, and root. Thus, MEA is an imprinted gene that displays parent-of-origin-dependent monoallelic expression specifically in the endosperm. These results suggest that the embryo abortion observed in mutant mea seeds is due, at least in part, to a defect in endosperm function. Silencing of the paternal MEA allele in the endosperm and the phenotype of mutant mea seeds supports the parental conflict theory for the evolution of imprinting in plants and mammals.     

A new assay for the analysis of X-chromosome inactivation based on methylation-specific PCR

The pattern of X-chromosome inactivation in females is currently evaluated by assays of differential methylation in the genes between the active and the inactive X chromosomes, with methylation-sensitive enzymes. We report a new assay in the human androgen receptor (HUMARA) locus involving a methylation-specific polymerase chain reaction (M-PCR) technique, independent of the use of restriction enzymes. The assay involves the chemical modification of DNA with sodium bisulfite and subsequent PCR. By using the assay with specific primers for the methylated allele, we obtained an X-inactivation pattern based on the ratio of the maternal inactive X to the paternal inactive X. These patterns were consistent with those obtained by conventional PCR assay at the same locus in 48 female cases. We also obtained another X-inactivation pattern based on the ratio of the maternal active X to the paternal active X by using specific primers for the unmethylated allele. The latter pattern was complementary to the former pattern, and a combination of these patterns produced a reliable X-inactivation pattern. The assay revealed that 12 (11%) of the 105 normal females had non-random inactivation patterns (>80:20 or <20:80). Four patients with an X; autosome translocation showed extremely non-random patterns, and these results were consistent with those obtained by previous molecular/cytogenetic studies. We conclude that M-PCR provides an accurate assay for X-inactivation and that it can be performed on various DNA samples unsuitable for restriction digestion. Received: 3 September 1998 / Accepted: 10 October 1998     

The mechanism of the mixed inheritance of chloroplast genes in Pelargonium

Summary The distributions are given of gene frequencies among embryos after G X W and W X G plastid crosses within and between eight Pelargonium cultivars and some of their inbred or hybrid derivatives.Two distinct segregation patterns are recognized. Homozygous type I female parents (Pr₁Pr₁) have a high frequency of progeny with only maternal alleles, are intermediate for biparental and low for paternal offspring. Heterozygous type II female plants (Pr₁Pr₂) have an equally high frequency of maternal and paternal offspring and a generally low biparental frequency. These correspond to L-shaped and U-shaped gene frequency distributions respectively in which the only modes are at 0 per cent (maternal embryos) and 100 per cent (paternal embryos), with no mode corresponding to the population mean and no sign of a Gaussian distribution.The extremely variable plastid gene frequencies are strongly influenced by the maternal nuclear genotype and by the plastid genotype in which the wild-type allele is always more successful than the mutant in strict comparisons.The relative frequencies of maternal and paternal zygotes, and the mean gene frequency among all the zygotes in a cross, are explicable in terms of the input frequencies of genes from the two parents, their degree of mixing, and by some form of selective replication of plastids. This selection is controlled by nuclear and plastid genotypes which may act in the same direction, to increase the frequency of either the maternal or the paternal alleles, or in opposition. But selection alone is inadequate to explain the shapes of the gene frequency distributions. Instead, a model is proposed in which the segregation or replication of plastids appears to have a strong random element, which results in random drift of gene frequencies within a heteroplasmic zygote or embryo.     

Methylation of HpaII and HhaI sites near the polymorphic CAG repeat in the human androgen-receptor gene correlates with X chromosome inactivation.

The human androgen-receptor gene (HUMARA; GenBank) contains a highly polymorphic trinucleotide repeat in the first exon. We have found that the methylation of HpaII and HhaI sites less than 100 bp away from this polymorphic short tandem repeat (STR) correlates with X inactivation. The close proximity of the restriction-enzyme sites to the STR allows the development of a PCR assay that distinguishes between the maternal and paternal alleles and identifies their methylation status. The accuracy of this assay was tested on (a) DNA from hamster/human hybrid cell lines containing either an active or inactive human X chromosome; (b) DNA from normal males and females; and (c) DNA from females showing nonrandom patterns of X inactivation. Data obtained using this assay correlated substantially with those obtained using the PGK, HPRT, and M27 beta probes, which detect X inactivation patterns by Southern blot analysis. In order to demonstrate one application of this assay, we examined X inactivation patterns in the B lymphocytes of potential and obligate carriers of X-linked agammaglobulinemia.     

鲤鲫人工多倍体谱系中同工酶和蛋白的基因表达

通过对红鲤、红鲫、镜鲤、鲤鲫杂种二倍体一代,二代,鲤鲫杂种三倍体,鲤鲫复合三倍体,鲤鲫杂种四倍体一代,二代的同工酶及蛋白电泳谱型和扫描数据分析表明,在鲤鲫人工多倍体谱系中,亲代的等位基因在杂交子代中共有四种表达模式;（1）两亲本基因在子代中共同表达,即共显表达;（2）父本的基因表达受到部分或完全的抑制,即母本的基因优先得到表达;（3）母本的基因表达受到抑制,父本的基因得到表达;（4）双亲本的基因表达均受到一定程度的抑制或都不表达。其中第一种表达模式是主要的模式。根据以上基因在杂交子代中的表达特点,可用同工酶和蛋白电泳图谱将鲤鲫人工多倍体谱系的各种生物型逐一加以区分。     

Increased Maternal Genome Dosage Bypasses the Requirement of the FIS Polycomb Repressive Complex 2 in Arabidopsis Seed Development

Seed development in flowering plants is initiated after a double fertilization event with two sperm cells fertilizing two female gametes, the egg cell and the central cell, leading to the formation of embryo and endosperm, respectively. In most species the endosperm is a polyploid tissue inheriting two maternal genomes and one paternal genome. As a consequence of this particular genomic configuration the endosperm is a dosage sensitive tissue, and changes in the ratio of maternal to paternal contributions strongly impact on endosperm development. The FERTILIZATION INDEPENDENT SEED (FIS) Polycomb Repressive Complex 2 (PRC2) is essential for endosperm development; however, the underlying forces that led to the evolution of the FIS-PRC2 remained unknown. Here, we show that the functional requirement of the FIS-PRC2 can be bypassed by increasing the ratio of maternal to paternal genomes in the endosperm, suggesting that the main functional requirement of the FIS-PRC2 is to balance parental genome contributions and to reduce genetic conflict. We furthermore reveal that the AGAMOUS LIKE (AGL) gene AGL62 acts as a dosage-sensitive seed size regulator and that reduced expression of AGL62 might be responsible for reduced size of seeds with increased maternal genome dosage.     

Sex differences in life history drive evolutionary transitions among maternal,paternal, and bi‐parental care

Evolutionary transitions among maternal, paternal, and bi‐parental care have been common in many animal groups. We use a mathematical model to examine the effect of male and female life‐history characteristics (stage‐specific maturation and mortality) on evolutionary transitions among maternal, paternal, and bi‐parental care. When males and females are relatively similar – that is, when females initially invest relatively little into eggs and both sexes have similar mortality and maturation – transitions among different patterns of care are unlikely to be strongly favored. As males and females become more different, transitions are more likely. If females initially invest heavily into eggs and this reduces their expected future reproductive success, transitions to increased maternal care (paternal → maternal, paternal → bi‐parental, bi‐parental → maternal) are favored. This effect of anisogamy (i.e., the fact that females initially invest more into each individual zygote than males) might help explain the predominance of maternal care in nature and differs from previous work that found no effect of anisogamy on the origin of different sex‐specific patterns of care from an ancestral state of no care. When male mortality is high or male egg maturation rate is low, males have reduced future reproductive potential and transitions to increased paternal care (maternal → paternal, bi‐parental → paternal, maternal → bi‐parental) are favored. Offspring need (i.e., low offspring survival in the absence of care) also plays a role in transitions to paternal care. In general, basic life‐history differences between the sexes can drive evolutionary transitions among different sex‐specific patterns of care. The finding that simple life‐history differences can alone lead to transitions among maternal and paternal care suggests that the effect of inter‐sexual life‐history differences should be considered as a baseline scenario when attempting to understand how other factors (mate availability, sex differences in the costs of competing for mates) influence the evolution of parental care.