Seed development: Early paternal gene activity in Arabidopsis.

Both parental genomes are expressed during embryogenesis, although the time of activation of the paternally inherited genes varies between organisms. Results reported by Vielle-Calzada et al. have suggested that delayed activation of the paternal genome seems to be the rule in plant development. We find, however, that during early embryogenesis in Arabidopsis, paternal genes are expressed and are sufficient for normal development. Our findings indicate that there is no overall maternal control of early embryogenesis, and that the contribution of the parental alleles needs to be assessed for each gene individually.     

The role of Tet3 DNA dioxygenase in epigenetic reprogramming by oocytes

Sperm and eggs carry distinctive epigenetic modifications that are adjusted by reprogramming after fertilization. The paternal genome in a zygote undergoes active DNA demethylation before the first mitosis. The biological significance and mechanisms of this paternal epigenome remodelling have remained unclear. Here we report that, within mouse zygotes, oxidation of 5-methylcytosine (5mC) occurs on the paternal genome, changing 5mC into 5-hydroxymethylcytosine (5hmC). Furthermore, we demonstrate that the dioxygenase Tet3 (ref. 5) is enriched specifically in the male pronucleus. In Tet3-deficient zygotes from conditional knockout mice, paternal-genome conversion of 5mC into 5hmC fails to occur and the level of 5mC remains constant. Deficiency of Tet3 also impedes the demethylation process of the paternal Oct4 and Nanog genes and delays the subsequent activation of a paternally derived Oct4 transgene in early embryos. Female mice depleted of Tet3 in the germ line show severely reduced fecundity and their heterozygous mutant offspring lacking maternal Tet3 suffer an increased incidence of developmental failure. Oocytes lacking Tet3 also seem to have a reduced ability to reprogram the injected nuclei from somatic cells. Therefore, Tet3-mediated DNA hydroxylation is involved in epigenetic reprogramming of the zygotic paternal DNA following natural fertilization and may also contribute to somatic cell nuclear reprogramming during animal cloning.     

Bypassing genomic imprinting allows seed development

In developing progeny of mammals the two parental genomes are differentially expressed according to imprinting marks, and embryos with only a uniparental genetic contribution die. Gene expression that is dependent on the parent of origin has also been observed in the offspring of flowering plants, and mutations in the imprinting machinery lead to embryonic lethality, primarily affecting the development of the endosperm-a structure in the seed that nourishes the embryo, analogous to the function of the mammalian placenta. Here we have generated Arabidopsis thaliana seeds in which the endosperm is of uniparental, that is, maternal, origin. We demonstrate that imprinting in developing seeds can be bypassed and viable albeit smaller seedlings can develop from seeds lacking a paternal contribution to the endosperm. Bypassing is only possible if the mother is mutant for any of the FIS-class genes, which encode Polycomb group chromatin-modifying factors. Thus, these data provide functional evidence that the action of the FIS complex balances the contribution of the paternal genome. As flowering plants have evolved a special reproduction system with a parallel fusion of two female with two male gametes, our findings support the hypothesis that only with the evolution of double fertilization did the action of the FIS genes become a requirement for seed development. Furthermore, our data argue for a gametophytic origin of endosperm in flowering plants, thereby supporting a hypothesis raised in 1900 by Eduard Strasburger.     

A global disorder of imprinting in the human female germ line

Imprinted genes are expressed differently depending on whether they are carried by a chromosome of maternal or paternal origin. Correct imprinting is established by germline-specific modifications; failure of this process underlies several inherited human syndromes. All these imprinting control defects are cis-acting, disrupting establishment or maintenance of allele-specific epigenetic modifications across one contiguous segment of the genome. In contrast, we report here an inherited global imprinting defect. This recessive maternal-effect mutation disrupts the specification of imprints at multiple, non-contiguous loci, with the result that genes normally carrying a maternal methylation imprint assume a paternal epigenetic pattern on the maternal allele. The resulting conception is phenotypically indistinguishable from an androgenetic complete hydatidiform mole, in which abnormal extra-embryonic tissue proliferates while development of the embryo is absent or nearly so. This disorder offers a genetic route to the identification of trans-acting oocyte factors that mediate maternal imprint establishment.     

The genetic basis of family conflict resolution in mice

Asymmetries in the costs and benefits of parental investment for mothers, fathers and offspring result in family conflict over the production and provisioning of young. In species where females provide most resources before and after birth, the resolution of this conflict may be influenced by genes expressed in mothers and by maternally and paternally inherited genes expressed in offspring. Here we disentangle these effects by means of reciprocal mating and cross-fostering of litters between two strains of mice that differ with respect to the typical resolution of family conflict. We find that differences in litter size between these two strains are determined by paternal genotype, whereas differences in provisioning are under maternal control, showing that there is antagonistic coadaptation of maternal and paternal effects on distinct life-history traits. Maternal provisioning is also influenced by the type of foster offspring. Contradictory to theoretical expectations, however, we find no evidence for a negative correlation across strains between maternal provisioning and offspring demand. Instead, we show that there is positive coadaptation such that offspring obtain more resources from foster mothers of the same strain as their natural mother, irrespective of their father's strain.     

An extrachromosomal factor causing loss of paternal chromosomes

Extrachromosomal inheritance is ubiquitous among plants and animals; however, most extrachromosomal factors are uniparentally inherited through females, but not through males. Examples include chloroplasts, mitochondria and a variety of intracellular symbionts. The only known exception to maternal extrachromosomal inheritance in an animal is a paternally transmitted sex ratio factor (psr) which causes all-male families in the parasitic wasp, Nasonia vitripennis. Normally in this wasp, male offspring are haploid and develop from unfertilized eggs whereas females are diploid and develop from fertilized eggs. The psr factor is either a venereally transmitted infection which prevents egg fertilization (and therefore causes all-male families), or a factor transmitted to eggs by the sperm of males carrying psr, which somehow prevents incorporation of the paternal chromosomes. Here we report that sperm from psr males fertilizes eggs, but that the paternal chromosomes are subsequently condensed into a chromatin mass before the first mitotic division of the egg and do not participate in further divisions. Resulting haploid offspring are male, but have inherited the paternal factor. This extrachromosomal factor promotes its own transmission at the expense of the paternal chromosomes, and therefore can be considered a 'selfish' genetic element.     

Birth of parthenogenetic mice that can develop to adulthood

Only mammals have relinquished parthenogenesis, a means of producing descendants solely from maternal germ cells. Mouse parthenogenetic embryos die by day 10 of gestation. Bi-parental reproduction is necessary because of parent-specific epigenetic modification of the genome during gametogenesis. This leads to unequal expression of imprinted genes from the maternal and paternal alleles. However, there is no direct evidence that genomic imprinting is the only barrier to parthenogenetic development. Here we show the development of a viable parthenogenetic mouse individual from a reconstructed oocyte containing two haploid sets of maternal genome, derived from non-growing and fully grown oocytes. This development was made possible by the appropriate expression of the Igf2 and H19 genes with other imprinted genes, using mutant mice with a 13-kilobase deletion in the H19 gene as non-growing oocytes donors. This full-term development is associated with a marked reduction in aberrantly expressed genes. The parthenote developed to adulthood with the ability to reproduce offspring. These results suggest that paternal imprinting prevents parthenogenesis, ensuring that the paternal contribution is obligatory for the descendant.     

Genomic imprinting determines methylation of parental alleles in transgenic mice

Mouse embryogenesis relies on the presence of both the maternal and the paternal genome for development to term. It has been proposed that specific modifications are imprinted onto the chromosomes during gametogenesis; these modifications are stably propagated, and their expression results in distinct and complementary contributions of the two parental genomes to the development of the embryo and the extraembryonic membranes. Genetic data further suggest that a substantial proportion of the genome could be subject to chromosomal imprinting, the molecular nature of which is unknown. We used random DNA insertions in transgenic mice to probe the genome for modified regions. The DNA methylation patterns of transgenic alleles were compared after transmission from mother or father in seven mouse strains carrying autosomal insertions of the same transgenic marker. One of these loci showed a clear difference in DNA methylation specific for its parental origin, with the paternally inherited copy being relatively undermethylated. This difference was observed in embryos on day 10 of gestation, but not in their extraembryonic membranes. Moreover, the methylation pattern was faithfully reversed upon each germline transmission to the opposite sex. Our findings provide evidence for heritable molecular differences between maternally and paternally derived alleles on mouse chromosomes.     

Crucial function of histone lysine methylation in plant reproduction

Unlike animals, plants do not set aside germ cells early in development. In angiosperm species, reproduction occurs in the adult plant upon flowering. The multicellular male and female gametophytes differentiate from meiotic products within reproductive floral organs. Double fertilization is another remarkable feature of most angiosperm species. The zygote derived from fertilization of the egg cell by one of the sperm cells and the endosperm from fertilization of the central cell by the second sperm cell develop in a coordinated manner together and enclosed in the sporophytic maternal integuments, forming the seed. Understanding plant reproduction is biologically pertinent and agronomically and ecologically important. Here, we describe the known functions of histone lysine methylations in various steps of reproduction in the reference plant Arabidopsis thaliana. It is emerging that histone lysine methylation is key for understanding epigenetic regulation networks of genome function.     

Methylation of a CTCF-dependent boundary controls imprinted expression of the Igf2 gene

The expression of the insulin-like growth factor 2 (Igf2) and H19 genes is imprinted. Although these neighbouring genes share an enhancer, H19 is expressed only from the maternal allele, and Igf2 only from the paternally inherited allele. A region of paternal-specific methylation upstream of H19 appears to be the site of an epigenetic mark that is required for the imprinting of these genes. A deletion within this region results in loss of imprinting of both H19 and Igf2 (ref. 5). Here we show that this methylated region contains an element that blocks enhancer activity. The activity of this element is dependent upon the vertebrate enhancer-blocking protein CTCF. Methylation of CpGs within the CTCF-binding sites eliminates binding of CTCF in vitro, and deletion of these sites results in loss of enhancer-blocking activity in vivo, thereby allowing gene expression. This CTCF-dependent enhancer-blocking element acts as an insulator. We suggest that it controls imprinting of Igf2. The activity of this insulator is restricted to the maternal allele by specific DNA methylation of the paternal allele. Our results reveal that DNA methylation can control gene expression by modulating enhancer access to the gene promoter through regulation of an enhancer boundary.     

植物激素调控籽粒大小的研究进展

植物种子的大小和品质是影响农作物产量的主要因素之一,研究控制种子籽粒大小发育的相关因素,对提高农作物产量具有重要意义.近年通过分析种子发育缺陷突变体或QTL等分子遗传学的方法,发现许多控制种子发育的重要基因影响着种子的大小和产量.对模式植物拟南芥和水稻的研究发现,许多调控种子发育的功能基因通过整合到植物激素的代谢或信号转导途径起作用,说明植物激素在调控籽粒发育中发挥重要作用,但有关作用的分子机理及其遗传调控网络待阐明.该文以模式植物拟南芥和水稻种子发育研究为例,着重介绍植物激素调控种子籽粒大小调控的研究进展.     

Role of paternal and maternal genomes in mouse development

There has been much speculation on whether mammalian eggs with two male pronuclei can develop normally. Eggs with two female pronuclei can sometimes develop as far as the 25-somite stage but with only very meagre extraembryonic tissues. We suggested that the genome undergoes specific imprinting during gametogenesis and that some paternal genes may be necessary for normal development of the extraembryonic tissues, in which only the maternal X chromosome remains active. However, the need for the maternal genome for development to term is not yet unequivocally established. The detailed study described here demonstrates that while between 40 and 50% of heterozygous reconstituted eggs with a male and a female pronucleus develop to term, none of the eggs with two male pronuclei does so. Furthermore, embryos in the latter case are very retarded, even though the trophoblast develops relatively well compared with embryos having two female pronuclei. Our combined results indicate that while the paternal genome is essential for the normal development of extraembryonic tissues, the maternal genome may be essential for some stages of embryogenesis.     

Non-invasive prenatal measurement of the fetal genome

The vast majority of prenatal genetic testing requires invasive sampling. However, this poses a risk to the fetus, so one must make a decision that weighs the desire for genetic information against the risk of an adverse outcome due to hazards of the testing process. These issues are not required to be coupled, and it would be desirable to discover genetic information about the fetus without incurring a health risk. Here we demonstrate that it is possible to non-invasively sequence the entire prenatal genome. Our results show that molecular counting of parental haplotypes in maternal plasma by shotgun sequencing of maternal plasma DNA allows the inherited fetal genome to be deciphered non-invasively. We also applied the counting principle directly to each allele in the fetal exome by performing exome capture on maternal plasma DNA before shotgun sequencing. This approach enables non-invasive exome screening of clinically relevant and deleterious alleles that were paternally inherited or had arisen as de novo germline mutations, and complements the haplotype counting approach to provide a comprehensive view of the fetal genome. Non-invasive determination of the fetal genome may ultimately facilitate the diagnosis of all inherited and de novo genetic disease.     

Preferential mutation of paternally derived RB gene as the initial event in sporadic osteosarcoma

Successive loss of function of both alleles of the retinoblastoma susceptibility gene (RB) on human chromosome 13 seems to be critical in the development of retinoblastoma and osteosarcoma. In cases where the tumour is familial and susceptibility is inherited, a mutation in one of the alleles is carried in the germline. We have recently shown that cytogenetically visible germline mutations are usually in the paternally derived gene. Such a bias would not be expected for sporadic (non-familial) tumours, where both mutations occur in somatic tissue, but there has been some indication of a bias towards initial somatic mutation in the paternally derived gene on chromosome 11 in sporadic Wilms tumour. We have now examined 13 sporadic osteosarcomas and find evidence which indicates that in 12 cases the initial mutation was in the paternal gene, suggesting the involvement of germinal imprinting in producing the differential susceptibility of the two genes to mutation.     

Distinct physiological and behavioural functions for parental alleles of imprinted Grb10

Imprinted genes, defined by their preferential expression of a single parental allele, represent a subset of the mammalian genome and often have key roles in embryonic development, but also postnatal functions including energy homeostasis and behaviour. When the two parental alleles are unequally represented within a social group (when there is sex bias in dispersal and/or variance in reproductive success), imprinted genes may evolve to modulate social behaviour, although so far no such instance is known. Predominantly expressed from the maternal allele during embryogenesis, Grb10 encodes an intracellular adaptor protein that can interact with several receptor tyrosine kinases and downstream signalling molecules. Here we demonstrate that within the brain Grb10 is expressed from the paternal allele from fetal life into adulthood and that ablation of this expression engenders increased social dominance specifically among other aspects of social behaviour, a finding supported by the observed increase in allogrooming by paternal Grb10-deficient animals. Grb10 is, therefore, the first example of an imprinted gene that regulates social behaviour. It is also currently alone in exhibiting imprinted expression from each of the parental alleles in a tissue-specific manner, as loss of the peripherally expressed maternal allele leads to significant fetal and placental overgrowth. Thus Grb10 is, so far, a unique imprinted gene, able to influence distinct physiological processes, fetal growth and adult behaviour, owing to actions of the two parental alleles in different tissues.