Polyandry, life-history trade-offs and the evolution of imprinting at Mendelian loci

Genomic imprinting causes parental origin-dependent differential expression of a small number of genes in mammalian and angiosperm plant embryos, resulting in non-Mendelian inheritance of phenotypic traits. The "conflict" theory of the evolution of imprinting proposes that reduced genetic relatedness of paternally, relative to maternally, derived alleles in offspring of polygamous females supports parental sex-specific selection at gene loci that influence maternal investment. While the theory's physiological predictions are well supported by observation, the requirement of polyandry in the evolution of imprinting from an ancestral Mendelian state has not been comprehensively analyzed. Here, we use diallelic models to examine the influence of various degrees of polyandry on the evolution of both Mendelian and imprinted autosomal gene loci that influence trade-offs between maternal fecundity and offspring viability. We show that, given a plausible assumption on the physiological relationship between maternal fecundity and offspring viability, low levels of polyandry are sufficient to reinforce exclusively the fixation of "greedy" paternally imprinted alleles that increase offspring viability at the expense of maternal fecundity and "thrifty" maternally imprinted alleles of opposite effect. We also show that, for all levels of polyandry, Mendelian alleles at genetic loci that influence the trade-off between maternal fecundity and offspring viability reach an evolutionary stable state, whereas pairs of reciprocally imprinted alleles do not.     

Strain-Dependent Developmental Relaxation of Imprinting of an Endogenous Mouse Gene, Kvlqt1

Genomic imprinting is an epigenetic modification of the gamete or zygote leading to parental origin-specific differential expression of the two alleles of a gene in somatic cells of the offspring. We previously reported that the human K_VLQT1 gene is imprinted and disrupted in patients with germline balanced chromosomal rearrangements and Beckwith–Wiedemann syndrome. In human, the gene is imprinted in most fetal tissues except the heart, and K_VLQT1 is part of a 1-Mb cluster of imprinted genes on human chromosome 11p15.5. We sought to determine whether the mouse K_vlqt1 gene is imprinted, by performing interspecific crosses of 129/SvEv mice with CAST/E_i(Mus musculus castaneus). We identified a transcribed polymorphism that distinguishes the two parental alleles in F₁offspring. Examination of embryonic, neonatal, and postnatal tissues revealed that K_vlqt1 is imprinted in mouse early embryos, in both female 129 × male CS and female CS × male 129 offspring, with preferential expression of the maternal allele, like the human homologue. Surprisingly, imprinting was developmentally relaxed, and the developmental stage and tissue specificity of relaxation of imprinting was strain-dependent. To our knowledge, this is the first example of an endogenous gene that shows strain-dependent developmental relaxation of imprinting.     

PGC7/Stella protects against DNA demethylation in early embryogenesis

DNA methylation is an important means of epigenetic gene regulation and must be carefully controlled as a prerequisite for normal early embryogenesis. Although global demethylation occurs soon after fertilization, it is not evenly distributed throughout the genome. Genomic imprinting and epigenetic asymmetry between parental genomes, that is, delayed demethylation of the maternal genome after fertilization, are clear examples of the functional importance of DNA methylation. Here, we show that PGC7/Stella, a maternal factor essential for early development, protects the DNA methylation state of several imprinted loci and epigenetic asymmetry. After determining that PGC7/Stella binds to Ran binding protein 5 (RanBP5; a nuclear transport shuttle protein), mutant versions of the two proteins were used to examine exactly when and where PGC7/Stella functions within the cell. It is likely that PGC7/Stella protects the maternal genome from demethylation only after localizing to the nucleus, where it maintains the methylation of several imprinted genes. These results demonstrate that PGC7/Stella is indispensable for the maintenance of methylation involved in epigenetic reprogramming after fertilization.     

Brain-expressed imprinted genes and adult behaviour: the example of Nesp and Grb10

Imprinted genes are defined by their parent-of-origin-specific monoallelic expression. Although the epigenetic mechanisms regulating imprinted gene expression have been widely studied, their functional importance is still unclear. Imprinted genes are associated with a number of physiologies, including placental function and foetal growth, energy homeostasis, and brain and behaviour. This review focuses on genomic imprinting in the brain and on two imprinted genes in particular, Nesp and paternal Grb10, which, when manipulated in animals, have been shown to influence adult behaviour. These two genes are of particular interest as they are expressed in discrete and overlapping neural regions, recognised as key “imprinting hot spots” in the brain. Furthermore, these two genes do not appear to influence placental function and/or maternal provisioning of offspring. Consequently, by understanding their behavioural function we may begin to shed light on the evolutionary significance of imprinted genes in the adult brain, independent of the recognised role in maternal care. In addition, we discuss the potential future directions of research investigating the function of these two genes and the behavioural role of imprinted genes more generally.     

Gene interactions in the evolution of genomic imprinting

Numerous evolutionary theories have been developed to explain the epigenetic phenomenon of genomic imprinting. Here, we explore a subset of theories wherein non-additive genetic interactions can favour imprinting. In the simplest genic interaction—the case of underdominance—imprinting can be favoured to hide effectively low-fitness heterozygous genotypes; however, as there is no asymmetry between maternally and paternally inherited alleles in this model, other means of enforcing monoallelic expression may be more plausible evolutionary outcomes than genomic imprinting. By contrast, more successful interaction models of imprinting rely on an asymmetry between the maternally and paternally inherited alleles at a locus that favours the silencing of one allele as a means of coordinating the expression of high-fitness allelic combinations. For example, with interactions between autosomal loci, imprinting functionally preserves high-fitness genotypes that were favoured by selection in the previous generation. In this scenario, once a focal locus becomes imprinted, selection at interacting loci favours a matching imprint. Uniparental transmission generates similar asymmetries for sex chromosomes and cytoplasmic factors interacting with autosomal loci, with selection favouring the expression of either maternal or paternally derived autosomal alleles depending on the pattern of transmission of the uniparentally inherited factor. In a final class of models, asymmetries arise when genes expressed in offspring interact with genes expressed in one of its parents. Under such a scenario, a locus evolves to have imprinted expression in offspring to coordinate the interaction with its parent''s genome. We illustrate these models and explore key links and differences using a unified framework.     

Genomic imprinting effects in a compromised in utero environment: Implications for a healthy pregnancy

Genomic imprinting in gametogenesis marks a subset of mammalian genes for parent-of-origin-dependent monoallelic expression in the offspring. In mice, the identification and manipulation of individual imprinted genes has shown that the diverse products of these genes are largely devoted to controlling pre- and postnatal growth. Human syndromes with parental origin effects have been characterized both at the phenotypic and genotypic levels, allowing further elucidation of the function and regulation of imprinted genes. Evidence suggests that a compromised in utero environment influences fetal growth through the modulation of epigenetic states. However it is not known whether imprinted genes, by their nature, might be more or less susceptible to such environmental influences. Here we review the progress made in addressing the influence of a compromised in utero environment on the behavior of imprinted genes. We also examine whether these environmental influences may have an impact on the later development of human disease.     

生殖细胞及早期胚胎基因组印记的表观调控

基因组印记是一种区别父母等位基因的表观遗传过程,可导致父源和母源基因特异性表达。印记是在配子发生过程中全基因组表观重编程时获得的,且在早期胚胎发育过程中得以维持。因此,在全基因组重编程过程中,对印记的识别和维持十分重要。本文概述了原始生殖细胞的印记清除、双亲原始生殖细胞的印记获得以及早期胚胎发育过程中印记维持的相关过程,并对在印记区域内保护印记基因免受全基因组DNA去甲基化的表观遗传因子的相关作用机制进行了讨论。     

A MODEL FOR GENOMIC IMPRINTING IN THE SOCIAL BRAIN: JUVENILES

What are imprinted genes doing in the adult brain? Genomic imprinting is when a gene's expression depends upon parent of origin. According to the prevailing view, the “kinship theory” of genomic imprinting, this effect is driven by evolutionary conflicts between genes inherited via sperm versus egg. This theory emphasizes conflicts over the allocation of maternal resources, and focuses upon genes that are expressed in the placenta and infant brain. However, there is growing evidence that imprinted genes are also expressed in the juvenile and adult brain, after cessation of parental care. These genes have recently been suggested to underpin neurological disorders of the social brain such as psychosis and autism. Here we advance the kinship theory by developing an evolutionary model of genomic imprinting for social behavior beyond the nuclear family. We consider the role of demography and mating system, emphasizing the importance of sex differences in dispersal and variance in reproductive success. We predict that, in hominids and birds, altruism will be promoted by paternally inherited genes and egoism will be promoted by maternally inherited genes. In nonhominid mammals we predict more diversity, with some mammals showing the same pattern and other showing the reverse. We discuss the implications for the evolution of psychotic and autistic spectrum disorders in human populations with different social structures.     

Genomic imprinting in plants: observations and evolutionary implications

The epigenetic phenomenon of genomic imprinting occurs among both plants and animals. In species where imprinting is observed, there are parent-of-origin effects on the expression of imprinted genes in offspring. This review focuses on imprinting in plants with examples from maize, where gene imprinting was first described, and Arabidopsis. Our current understanding of imprinting in plants is presented in the context of cytosine methylation and imprinting in mammals, where developmentally essential genes are imprinted. Important considerations include the structure and organization of imprinted genes and the role of regional, differential methylation. Imprinting in plants may be related to other epigenetic phenomena including paramutation and transgene silencing. Finally, we discuss the role of gene structure and evolutionary implications of imprinting in plants.     

Testing the imprinted brain: parent-of-origin effects on empathy and systemizing

Genomic imprinting is a violation of Mendel's laws that enables selection to act on genes, depending on parent of origin, but, even more controversially, on the sex of the offspring. This study tested whether there are parent-of-origin effects on the heritability of empathy and systemizing in the general population as part of a larger question concerning the role of imprinted genes in the evolution of human cognition and behaviour. The measures tested were the Empathy and Systemizing Quotients as proxies for the related terms mentalistic and mechanistic cognition in the imprinted brain theory.To test genomic imprinting hypotheses, correlations in behavioural scores between pairs of full, maternal and paternal siblings were compared. Where scores are influenced by imprinted genes, the actual correlations between pairs of siblings will differ from those expected following classical Mendelian inheritance in a predictable way depending on what kind of imprinting is influencing the trait. These theoretical predictions were used to test the fit of the data against Mendelian and imprinting models using structural equation modeling. The imprinted brain theory proposes a trade-off between maternally influenced mentalistic cognition and paternally influenced mechanistic cognition. However, the results of this study support a model of contrasting maternal and paternal influences on strong and weak empathizing and a maternal influence on systemizing. Although the sample size was insufficient to comprehensively analyse sex-limitation models, there is some evidence that heritability of systemizing is stronger in females than in males.     

Chromatin regulators of genomic imprinting

Genomic imprinting is an epigenetic phenomenon in which genes are expressed monoallelically in a parent-of-origin-specific manner. Each chromosome is imprinted with its parental identity. Here we will discuss the nature of this imprinting mark. DNA methylation has a well-established central role in imprinting, and the details of DNA methylation dynamics and the mechanisms that target it to imprinted loci are areas of active investigation. However, there is increasing evidence that DNA methylation is not solely responsible for imprinted expression. At the same time, there is growing appreciation for the contributions of post-translational histone modifications to the regulation of imprinting. The integration of our understanding of these two mechanisms is an important goal for the future of the imprinting field. This article is part of a Special Issue entitled: Chromatin and epigenetic regulation of animal development.     

Multilocus epimutations of imprintome in the pathology of human embryo development

Genomic imprinting is one of the key epigenetic phenomena involved in embryonic development of eutherians and humans. Molecular mechanisms of imprinting disturbances in the pathology of prenatal and postnatal onthogenesis are to a great extent related to methylation abnormalities of the imprinted genes. Over recent years, data are accumulating on multiple abnormalities of methylation simultaneously in several imprinted loci in the development of various pathologies that raises the issue of deciphering the structural and functional organization of imprintome and the interaction of imprinted genes. The present work analyzes DNA methylation of 51 imprinted genes in the placental tissues of human spontaneous abortions. We revealed multiple epimutations in from four to 12 imprinted genes in every embryo. Most of the epimutations (78%) were of a postzygotic origin. It has been established for the first time that the total incidence of methylation disturbance in maternal and paternal alleles of the imprinted genes leading to embryo development suppression is significantly higher than the incidence of epimutations, which stimulate embryogenesis. This fact supports at the epigenetic level the hypothesis of parent-offspring conflict that describes the occurrence of a monoallelic expression of imprinted genes in mammalian evolution.     

Genomic imprinting and endosperm development in flowering plants

Genomic imprinting, the parent-of-origin-specific expression of genes, plays an important role in the seed development of flowering plants. As different sets of genes are imprinted and hence silenced in maternal and paternal gametophyte genomes, the contributions of the parental genomes to the offspring are not equal. Imbalance between paternally and maternally imprinted genes, for instance as a result of interploidy crosses, or in seeds in which imprinting has been manipulated, results in aberrant seed development. It is predominantly the endosperm, and not or to a far lesser extent the embryo, that is affected by such imbalance. Deviation from the normal 2m:1p ratio in the endosperm genome has a severe effect on endosperm development, and often leads to seed abortion. Molecular expression data for imprinted genes suggest that genomic imprinting takes place only in the endosperm of the developing seed. Although far from complete, a picture of how imprinting operates in flowering plants has begun to emerge. Imprinted genes on either the maternal or paternal side are marked and silenced in a process involving DNA methylation and chromatin condensation. In addition, on the maternal side, imprinted genes are most probably under control of the polycomb FIS genes.     

Epigenetic asymmetry in the mammalian zygote and early embryo: relationship to lineage commitment?

Epigenetic asymmetry between parental genomes and embryonic lineages exists at the earliest stages of mammalian development. The maternal genome in the zygote is highly methylated in both its DNA and its histones and most imprinted genes have maternal germline methylation imprints. The paternal genome is rapidly remodelled with protamine removal, addition of acetylated histones, and rapid demethylation of DNA before replication. A minority of imprinted genes have paternal germline methylation imprints. Methylation and chromatin reprogramming continues during cleavage divisions, but at the blastocyst stage lineage commitment to inner cell mass (ICM) or trophectoderm (TE) fate is accompanied by a dramatic increase in DNA and histone methylation, predominantly in the ICM. This may set up major epigenetic differences between embryonic and extraembryonic tissues, including in X-chromosome inactivation and perhaps imprinting. Maintaining epigenetic asymmetry appears important for development as asymmetry is lost in cloned embryos, most of which have developmental defects, and in particular an imbalance between extraembryonic and embryonic tissue development.     

Multilocus epimutations of imprintome in the pathology of embryo development

Genomic imprinting is one of the most significant epigenetic phenomena, which is involved in the support of eutherians and human embryo development. Molecular mechanisms of imprinting disturbance in the pathology of pre- and postnatal ontogeny are related to a considerable degree to aberrant DNA methylation of imprinted genes. At present time data about multiple abnormalities of DNA methylation arising simultaneously in several imprinted loci are accumulated. This fact brings up the problem of interpretation of imprintome structural and functional organization, as well as interaction of imprinted genes. At present study DNA methylation analysis of 51 imprinted genes in placental tissues of human spontaneous abortions was performed. The presence of several epimutations affected from four to 12 imprinted genes was observed in each embryo. Majority of epimutations (78%) had a postzygotic origin. It was shown for the first time that the total incidence of abnormal DNA methylation of maternal and paternal alleles of imprinted genes, which lead to suppression of embryo development, is significantly higher than the incidence of epimutations, which can lead to stimulation of ontogenesis processes. This fact supports at the epigenetic level the "sex conflict" hypothesis, which explains the appearance of monoallelic imprinted genes expression in the evolution of mammals.     

Regulation and flexibility of genomic imprinting during seed development

Genomic imprinting results in monoallelic gene expression in a parent-of-origin-dependent manner. It is achieved by the differential epigenetic marking of parental alleles. Over the past decade, studies in the model systems Arabidopsis thaliana and maize (Zea mays) have shown a strong correlation between silent or active states with epigenetic marks, such as DNA methylation and histone modifications, but the nature of the primary imprint has not been clearly established for all imprinted genes. Phenotypes and expression patterns of imprinted genes have fueled the perception that genomic imprinting is specific to the endosperm, a seed tissue that does not contribute to the next generation. However, several lines of evidence suggest a potential role for imprinting in the embryo, raising questions as to how imprints are erased and reset from one generation to the next. Imprinting regulation in flowering plants shows striking similarities, but also some important differences, compared with the mechanisms of imprinting described in mammals. For example, some imprinted genes are involved in seed growth and viability in plants, which is similar in mammals, where imprinted gene regulation is essential for embryonic development. However, it seems to be more flexible in plants, as imprinting requirements can be bypassed to allow the development of clonal offspring in apomicts.     

Epigenetic Resetting of a Gene Imprinted in Plant Embryos

Genomic imprinting resulting in the differential expression of maternal and paternal alleles in the fertilization products has evolved independently in placental mammals and flowering plants. In most cases, silenced alleles carry DNA methylation [1]. Whereas these methylation marks of imprinted genes are generally erased and reestablished in each generation in mammals [2], imprinting marks persist in endosperms [3], the sole tissue of reported imprinted gene expression in plants. Here we show that the maternally expressed in embryo 1 (mee1) gene of maize is imprinted in both the embryo and endosperm and that parent-of-origin-specific expression correlates with differential allelic methylation. This epigenetic asymmetry is maintained in the endosperm, whereas the embryonic maternal allele is demethylated on fertilization and remethylated later in embryogenesis. This report of imprinting in the plant embryo confirms that, as in mammals, epigenetic mechanisms operate to regulate allelic gene expression in both embryonic and extraembryonic structures. The embryonic methylation profile demonstrates that plants evolved a mechanism for resetting parent-specific imprinting marks, a necessary prerequisite for parent-of-origin-dependent gene expression in consecutive generations. The striking difference between the regulation of imprinting in the embryo and endosperm suggests that imprinting mechanisms might have evolved independently in both fertilization products of flowering plants.     

基因组印记与疾病研究进展

基因组印记是一种特别的非孟德尔遗传现象,即来自双亲的等位基因在子代中的差异性表达,是遗传后的基因调控方式,主要与基因组甲基化模式有关,包括去甲基化、重新甲基化及甲基化维持三个过程。印记基因主要通过对启动子、边界元件及非编码RNA的作用来调控基因表达。基因组印记异常与一些先天性疾病相关,也与肿瘤发生和易感性有关,