Abnormal X-Chromosome Dosage Compensation as a Possible Cause of Early Developmental Failure in Mice. (X-chromosome inactivation/trophectoderm/imprinting/embryonic development)

Misdirection of dosage compensation underlies bidirectional sex-specific death in Wolbachia-infected Ostrinia scapulalis

Endosymbiotic bacteria of the genus Wolbachia often manipulate the reproductive system of their hosts to propagate themselves in host populations. Ostrinia scapulalis moths infected with Wolbachia (wSca) produce female-only progeny (sex chromosomes: ZW), whereas females cured of the infection by antibiotic treatment produce male-only progeny (ZZ). The occurrence of female- and male-only progeny has been attributed to the specific death of the opposite sex during embryonic and larval development. In this bidirectional sex-specific lethality, embryos destined to die express a phenotypic sex opposite to their genotypic sex. On the basis of these findings, we suggested that wSca carries a genetic factor that feminizes the male host, the W chromosome of the host has lost its feminizing function, and discordance between the genotypic and phenotypic sexes underlies this sex-specific death. In the present study, we examined whether the failure of dosage compensation was responsible for this sex-specific mortality. Quantitative PCRs showed that Z-linked gene expression levels in embryos destined to die were not properly dosage compensated; they were approximately two-fold higher in the male progeny of wSca-infected females and approximately two-fold lower in the female progeny of infected-and-cured females. These results support our hypothesis that misdirection of dosage compensation underlies the sex-specific death.

Difference in chromatin packaging between active and inactive X chromosomes by fractionation and allele-specific detection

Using a novel method consisting of chromatin fractionation and allele-specific detection, chromatin packaging is compared between active X (Xa) and inactive X (Xi) chromosomes for five tumor cell clones that were derived from inter-subspecific F1 female mice. Separation of heterochromatic (H) and euchromatic (E) fractions is monitored by hybridization with subtelomeric satellite DNA and ribosomal RNA gene and by PCR amplification of p53 gene/pseudogene with one primer set. The H fraction was enriched with satellite and p53 pseudogene probably existing in heterochromatic regions while the E fraction showed inverse, suggesting fair separation. Analysis with seven marker and three gene loci revealed concentration of alleles on Xi in the H fraction and those on Xa in the E fraction, though the concentration levels varied. This implies that the packaging level of Xi is higher than that of active or inactive euchromatin on Xa. Intriguingly, one cell line showed biallelic expression and chromatin relaxation of the Pgk-1 locus, suggesting that the relaxation occur regionally on X chromosome.

Genomic imprinting--defusing the ovarian time bomb

Why do mammals imprint their parental genomes? Imprinting is seen in many phyla, but that in mammals is by far the most dramatic. Is there something peculiar to mammals that calls for such a striking phenomenon? We propose that imprinting is a device that protects female mammals from the potential ravages of ovarian trophoblast disease. Without imprinting, the ovarian teratomas that frequently arise from parthenogenetically activated oocytes in situ might be capable of forming malignant trophoblast. An allele that favored imprinting would spread rapidly because of the great increase in fitness associated with suppressing a lethal cancer of females.

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

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

Variable X chromosome inactivation patterns in near-tetraploid murine EC x somatic cell hybrid cells differentiated in vitro

For the cytogenetic study of X chromosome inactivation as an X chromosome dosage compensation mechanism, we isolated a number of XXXX, XXX, and XXY near-tetraploid mouse hybrid cell clones by fusing XX or XO embryonal carcinoma cells with lymphocytes carrying a structurally altered X chromosome(s). The inactive X chromosome from the female lymphocyte was reactivated in these hybrid clones which retained embryonal carcinoma morphology so far as they were cultured on the collagen-coated plastic surface in the medium supplemented with leukemia inhibitory factor (LIF) and betamercaptoethanol (BME). Some of these clones developed balloon-like cystic embryoid bodies when they were allowed to form cell aggregates in medium without LIF and BME in bacteriological petri dishes to which they do not adhere. X chromosome inactivation occurring during this process detected by the incorporation of 5-bromodeoxyuridine did not conform to the expected pattern leaving two X chromosomes active in every tetraploid cells. This may suggest either that the X-inactivation mechanism evolved primarily, for the diploid cell is unable to deal with tetraploid conditions efficiently, or that the present system of in vitro differentiation represents an anomalous situation never encountered in vivo.

A paternally imprinted X chromosome retards the development of the early mouse embryo

It has previously been shown that XO mouse fetuses with a paternally derived X chromosome (Xp) are developmentally retarded and consequently smaller than their XX sibs, and that XX fetuses are retarded when compared with their XY sibs. The genetic basis for these early XO-XX and XX-XY differences has not been determined. Here we show that 10.5 day post coitum XO mouse fetuses with a maternal X chromosome, rather than being smaller than their XX sibs, are significantly larger and equivalent in size to their XY sibs. Thus the retardation of XpO fetuses must be due to an effect of their paternally derived X chromosome. The finding that XmO fetuses are larger than XX fetuses and equivalent in size to XY fetuses suggests that the XX-XY difference present at 10.5 days post coitum is largely due to the difference in X chromosome constitution rather than to a Ychromosome effect.

Site of action of imprinted genes revealed by phenotypic analysis of parthenogenetic embryos

The phenotypes of early post-implantation parthenogenetic embryos were examined. The spectrum of phenotypes suggested that three stages are adversely affected by imprinting--implantation, pregastrulation, and postgastrulation. Survival of parthenogenetic embryos past these developmental blocks can be improved but not completely overcome by experimental asynchrony. These results suggest that imprinting may be "leaky" at early stages.