Imprinting but not cytonuclear interactions affects parent-of-origin effect on seed size in Arabidopsis hybrids
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ABSTRACT: The parent-of-origin effect on seed size can result from imprinting or a combinational effect between cytoplasmic and nuclear genomes, but their relative contributions remain unknown. To discern these confounding effects, we generated cytoplasmic-nuclear substitution (CNS) lines using recurrent backcrossing in theArabidopsis thalianaecotypes Col-0 and C24. These CNS lines differ only in the nuclear genome (imprinting) or in the cytoplasm. The CNS reciprocal hybrids with the same cytoplasm display a ∼20% seed size difference as observed in the conventional hybrids. However, seed size is similar between the reciprocal cybrids with fixed imprinting. Transcriptome analyses in the endosperm of CNS hybrids using laser-capture microdissection have identified 104 maternally expressed genes (MEGs) and 90 paternally-expressed genes (PEGs). These imprinted genes are involved in pectin catabolism and cell wall modification in the endosperm.HDG9, an epiallele and one of 11 cross-specific imprinted genes, controls seed size. In the embryo, a handful of imprinted genes is found in the CNS hybrids but only one is expressed higher in the embryo than endosperm.AT4G13495encodes a long-noncoding RNA (lncRNA), but no obvious seed phenotype is observed in the lncRNA knockout lines.NRPD1, encoding the largest subunit of RNA Pol IV, is involved in the biogenesis of small interfering RNAs. Seed size and embryo is larger in the cross usingnrpd1as the maternal parent than in the reciprocal cross. In spite of limited ecotypes tested, these results suggest potential roles of imprinting andNRPD1-mediated small RNA pathway in seed size variation in hybrids.
Project description:Imprinted gene expression occurs during seed development in plants and is closely tied to differential DNA methylation of maternal and paternal alleles, particularly at proximal transposable elements (TEs). Since the epigenetic modification of TEs can vary within species, we investigated intraspecific variation in imprinting, coupled with analysis of DNA methylation and small RNAs, among three strains of Arabidopsis that display diverse seed size phenotypes. Unexpectedly we found that one strain, Cvi, is globally CG hypomethylated. We discovered three examples of strain-specific imprinting caused by epigenetic variation at a TE. Our data allowed us to predict and experimentally validate an instances of allele-specific imprinting in additional strains based only on methylation patterns. We conclude that numerous differences in imprinting can evolve in highly similar, recently diverged genotypes due to epiallelic variation present within the species. Our data demonstrate that epiallelic variation and genomic imprinting intersect to produce novel gene expression patterns in seeds. Examination of parent-of-origin specific and total gene expression in embryo, endosperm, and whole seeds. Samples with the same heading are biological replicates (e.g. CVN1, CVN2, and CVN3). High throughput Illumina sequencing of poly-A selected RNA from Arabidopsis Col, Ler and Cvi reciprocal F1 hybrid embryo and endosperm tissue isolated at 6 days after pollination to identify imprinted genes.
Project description:In many eukaryotes, reproduction involves contributions of genetic material from two parents. At some genes there are parent-of-origin differences in the expression of the maternal and paternal alleles of a gene and this is referred to as imprinting. The analysis of allele-specific expression in several maize hybrids allowed the comprehensive detection of imprinted genes. By comparing allelic expression patterns in multiple crosses, it was possible to observe allelic variation for imprinting in maize. The comparison of genes subject to imprinting in multiple plant species reveals limited conservation for imprinting. The subset of genes that exhibit conserved imprinting in maize and rice may play important, dosage-dependent roles in regulation of seed development. In this study, deep sequencing of RNA isolated from 14 days-after-pollination (DAP) endosperm tissue of five reciprocal hybrid pairs was performed to identify imprinted genes.
Project description:The embryo is responsible for transmitting genetic information to the next generation. However, the underlying gene expression and gene imprinting during early embryo development remain largely elusive in maize. Using high-throughput RNA sequencing, we analyzed the allelic gene expression patterns of maize embryos from reciprocal crosses between inbred lines B73 and Mo17 at six time points (3 to 13 days after pollination). A total of 9532 genes were found to be differentially expressed in developmental stage, 3512 of 9532 genes were affected by cross direction. Co-expression analysis uncovered the sequential gene activations during maize embryo development. Further, we found 64 embryo strongly imprinted genes, including 57 maternally expressed imprinted genes (MEG) and 7 paternally expressed imprinted genes (PEG) in the maize embryo. Among them, 20 genes were continuously imprinted and 36 imprinted genes were newly identified. By applying In situ hybridization, we verified that six of the differentially expressed genes showed enriched transcription in the embryo. In addition, mutant analyses indicated that three of the imprinted genes displayed reduced embryo size. Therefore, our data shed new light on our understanding of the gene expression and gene imprinting in early maize embryo development, and suggested that imprinted genes are important for proper embryo development.
Project description:Imprinted gene expression occurs during seed development in plants and is associated with differential DNA methylation of parental alleles, particularly at proximal transposable elements (TEs). Imprinting variability could contribute to observed parent-of-origin effects on seed development. We investigated intraspecific variation in imprinting, coupled with analysis of DNA methylation and small RNAs, among three Arabidopsis strains with diverse seed phenotypes. The majority of imprinted genes were parentally biased in the same manner among all strains. However, we identified several examples of allele-specific imprinting correlated with intraspecific epigenetic variation at a TE. We successfully predicted imprinting in additional strains based on methylation variability. We conclude that there is standing variation in imprinting even in recently diverged genotypes due to intraspecific epiallelic variation. These data demonstrate that epiallelic variation and genomic imprinting intersect to produce novel gene expression patterns in seeds. Whole genome bisulfite sequencing of embryo and endosperm (14 samples).
Project description:The discovery of genomic imprinting through studies of manipulated mouse embryos indicated that the paternal genome has a major influence on placental development. However, previous research has not demonstrated paternal bias in imprinted genes. We applied RNA sequencing to trophoblast tissue from reciprocal hybrids of horse and donkey, where genotypic differences allowed parent-of-origin identification of most expressed genes. Using this approach, we identified a core group of 15 ancient imprinted genes of which 10 were paternally expressed. An additional 78 candidate novel imprinted genes identified by RNA-seq also showed paternal bias. Pyrosequencing was used to confirm the imprinting status of six of the novel genes, including the insulin receptor (INSR), which may play a role in growth regulation with its reciprocally imprinted ligand, histone acetyltransferase (HAT1), the first example of an imprinted gene involved in chromatin modification, and LY6G6C, the first imprinted gene to be identified in the major histocompatibility complex. The 78 novel candidate imprinted genes displayed parent-of-origin expression bias in placenta but not fetus, and most showed less than 100% silencing of the imprinted allele. Some displayed variability in imprinting status among individuals. This results in a unique epigenetic signature for each placenta that contributes to variation in the intrauterine environment and thus presents the opportunity for natural selection to operate on parent-of-origin differential regulation. Taken together, these features highlight the plasticity of imprinting in mammals and the central importance of the placenta as a target tissue for genomic imprinting. Examine allelic expression from four individual samples of invasive trophoblast tissue of the chorionic girdle from gestation day 33 conceptuses of horse, donkey, mule and hinny.
Project description:High throughput Illumina sequencing of poly-A selected RNA from Arabidopsis Col and Ler reciprocal F1 hybrid embryo and endosperm tissue isolated at 6-7 days after pollination to identify imprinted genes. Examination of parent-of-origin specific and total gene expression in seed tissues.
Project description:Many questions about the regulation, functional specialization, computational prediction, and evolution of genomic imprinting would be better addressed by having an exhaustive genome-wide catalog of genes that display parent-of-origin differential expression. As a first-pass scan for novel imprinted genes, we performed mRNA-seq experiments on E17.5 mouse placenta cDNA samples from reciprocal cross F1 progeny of AKR and PWD mouse strains, and quantified the allele-specific expression and the degree of parent-of-origin effect transcriptome-wide. We confirmed the imprinting status of 23 known imprinted genes in the placenta, and found that 12 genes reported previously to be imprinted in other tissues are also imprinted in mouse placenta. Through a well-replicated design using an orthogonal technology, we verified five novel imprinted genes that are not known to be imprinted in mouse. It appears that most of the strongly imprinted genes have already been identified, at least in the placenta, and that evidence supports perhaps 100 additional weakly imprinted genes. Despite previous appearance that the placenta tends to display an excess of maternally-expressed imprinted genes, when the full set of genes is uniformly scored as in this study, this maternal bias disappeared. Examine allelic expression in E17.5 placenta tissues from two individual samples, one from each of the two reciprocal crosses.
Project description:The discovery of genomic imprinting through studies of manipulated mouse embryos indicated that the paternal genome has a major influence on placental development. However, previous research has not demonstrated paternal bias in imprinted genes. We applied RNA sequencing to trophoblast tissue from reciprocal hybrids of horse and donkey, where genotypic differences allowed parent-of-origin identification of most expressed genes. Using this approach, we identified a core group of 15 ancient imprinted genes of which 10 were paternally expressed. An additional 78 candidate novel imprinted genes identified by RNA-seq also showed paternal bias. Pyrosequencing was used to confirm the imprinting status of six of the novel genes, including the insulin receptor (INSR), which may play a role in growth regulation with its reciprocally imprinted ligand, histone acetyltransferase (HAT1), the first example of an imprinted gene involved in chromatin modification, and LY6G6C, the first imprinted gene to be identified in the major histocompatibility complex. The 78 novel candidate imprinted genes displayed parent-of-origin expression bias in placenta but not fetus, and most showed less than 100% silencing of the imprinted allele. Some displayed variability in imprinting status among individuals. This results in a unique epigenetic signature for each placenta that contributes to variation in the intrauterine environment and thus presents the opportunity for natural selection to operate on parent-of-origin differential regulation. Taken together, these features highlight the plasticity of imprinting in mammals and the central importance of the placenta as a target tissue for genomic imprinting.
Project description:In plants, imprinted gene expression occurs in endosperm seed tissue and can be associated with differential DNA methylation between maternal and paternal alleles. Imprinting is theorized to have been selected for because of conflict between parental genomes in offspring, but most studies of imprinting have been conducted in Arabidopsis thaliana, an inbred primarily self-fertilizing species that should have limited parental conflict. We examined embryo and endosperm allele-specific expression and DNA methylation genome-wide in the wild outcrossing species Arabidopsis lyrata. Here we show that the majority of A. lyrata imprinted genes exhibit parentally-biased expression in A. thaliana, suggesting that there is evolutionary conservation in gene imprinting. Surprisingly, we discovered substantial interspecies differences in methylation features associated with paternally expressed imprinted genes (PEGs). Unlike A. thaliana, the maternal allele of many A. lyrata PEGs was hypermethylated in the CHG context. Increased maternal allele CHG methylation was associated with increased expression bias in favor of the paternal allele. We propose that CHG methylation maintains or reinforces repression of maternal alleles of PEGs. These data suggest that while the genes subject to imprinting are largely conserved, there is flexibility in the epigenetic mechanisms employed between closely related species to maintain monoallelic expression. This supports the idea that imprinting of specific genes is a functional phenomenon, and not simply a byproduct of seed epigenomic reprogramming. Examination of total gene expression, parent-of-origin specific allelic bias, or DNA methylation in embryo, endosperm, flower bud or seedcoat tissue from Arabidopsis lyrata accessions MN47 (MN), Karhumaki (Kar or KA), and crosses between them. High-throughput Illumina poly-A-selected mRNA-seq was used to identify imprinted genes in A. lyrata, and high-throughput Illumina whole genome bisulfite-sequencing was used to examine DNA methylation. mRNA-seq samples are designated MMxFF_T# where MM is the mother of the cross (either MN for MN47 or KA for Kar), FF is the father, T is the tissue (E for embryo, N for endosperm, S for seedcoat, b for buds), and # is the replicate numbers. Samples obtained from bisulfite sequencing follow the same naming but have suffix _BS and indicate cytosine methylation context (CpG, CHG, or CHH). For KAxMN bisulfite sequencing, additional files MMxFF_T#_BS_P_C.txt follow the same naming scheme but contain context-specific methylation data (C) from reads that mapped preferentially to one parent strain (P).
Project description:Many questions about the regulation, functional specialization, computational prediction, and evolution of genomic imprinting would be better addressed by having an exhaustive genome-wide catalog of genes that display parent-of-origin differential expression. As a first-pass scan for novel imprinted genes, we performed mRNA-seq experiments on E17.5 mouse placenta cDNA samples from reciprocal cross F1 progeny of AKR and PWD mouse strains, and quantified the allele-specific expression and the degree of parent-of-origin effect transcriptome-wide. We confirmed the imprinting status of 23 known imprinted genes in the placenta, and found that 12 genes reported previously to be imprinted in other tissues are also imprinted in mouse placenta. Through a well-replicated design using an orthogonal technology, we verified five novel imprinted genes that are not known to be imprinted in mouse. It appears that most of the strongly imprinted genes have already been identified, at least in the placenta, and that evidence supports perhaps 100 additional weakly imprinted genes. Despite previous appearance that the placenta tends to display an excess of maternally-expressed imprinted genes, when the full set of genes is uniformly scored as in this study, this maternal bias disappeared.