BackgroundDNA methylation is an important feature of plant epigenomes, involved in the formation of heterochromatin and affecting gene expression. Extensive variation of DNA methylation patterns within a species has been uncovered from studies of natural variation. However, the extent to which DNA methylation varies between flowering plant species is still unclear. To understand the variation in genomic patterning of DNA methylation across flowering plant species, we compared single base resolution DNA methylomes of 34 diverse angiosperm species.ResultsBy analyzing whole-genome bisulfite sequencing data in a phylogenetic context, it becomes clear that there is extensive variation throughout angiosperms in gene body DNA methylation, euchromatic silencing of transposons and repeats, as well as silencing of heterochromatic transposons. The Brassicaceae have reduced CHG methylation levels and also reduced or loss of CG gene body methylation. The Poaceae are characterized by a lack or reduction of heterochromatic CHH methylation and enrichment of CHH methylation in genic regions. Furthermore, low levels of CHH methylation are observed in a number of species, especially in clonally propagated species.ConclusionsThese results reveal the extent of variation in DNA methylation in angiosperms and show that DNA methylation patterns are broadly a reflection of the evolutionary and life histories of plant species.Electronic supplementary materialThe online version of this article (doi:10.1186/s13059-016-1059-0) contains supplementary material, which is available to authorized users.

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In plants, CG DNA methylation is prevalent in the transcribed regions of many constitutively expressed genes (gene body methylation; gbM), but the origin and function of gbM remain unknown. Here we report the discovery that Eutrema salsugineum has lost gbM from its genome, to our knowledge the first instance for an angiosperm. Of all known DNA methyltransferases, only CHROMOMETHYLASE 3 (CMT3) is missing from E. salsugineum. Identification of an additional angiosperm, Conringia planisiliqua, which independently lost CMT3 and gbM, supports that CMT3 is required for the establishment of gbM. Detailed analyses of gene expression, the histone variant H2A.Z, and various histone modifications in E. salsugineum and in Arabidopsis thaliana epigenetic recombinant inbred lines found no evidence in support of any role for gbM in regulating transcription or affecting the composition and modification of chromatin over evolutionary timescales.DNA methylation | gene body methylation | epigenetics | histone modifications | CHROMOMETHYLASE 3 I n angiosperms, cytosine DNA methylation occurs in three sequence contexts: Methylated CG (mCG) is catalyzed by METHYLTRANSFERASE 1 (MET1), mCHG (where H is A/C/T) by CHROMOMETHYLASE 3 (CMT3), and mCHH by DOMAINS REARRANGED METHYLTRANSFERASE 2 (DRM2) or CHROMOMETHYLASE 2 (CMT2) (1). MET1 performs a maintenance function and is targeted by VARIANT IN METHYLATION 1 (VIM1), which binds preexisting hemimethylated CG sites. In contrast, DRM2 is targeted by RNA-directed DNA methylation (RdDM) for the de novo establishment of mCHH. CMT3 forms a self-reinforcing loop with the H3K9me2 pathway to maintain mCHG; however, considering that transformation of CMT3 into the cmt3 background can rescue DNA methylation defects, it is reasonable to also consider CMT3 a de novo methyltransferase (2). Two main lines of evidence suggest that DNA methylation plays an important role in the transcriptional silencing of transposable elements (TEs): that TEs are usually methylated, and that the loss of DNA methylation (e.g., in methyltransferase mutants) is often accompanied by TE reactivation.A large number of plant genes (e.g., ∼13.5% of all Arabidopsis thaliana genes) also contain exclusively mCG in the transcribed region and a depletion of mCG from both the transcriptional start and stop sites (referred to as "gene body DNA methylation"; gbM) ( Fig. 1A) (3)(4)(5). A survey of plant methylome data showed that the emergence of gbM in the plant kingdom is specific to angiosperms (6), whereas nonflowering plants (such as mosses and green algae) have much more diverse genic methylation patterns (7,8). Similar to mCG at TEs, the maintenance of gbM requires MET1. In contrast to DNA methylation at TEs, however, gbM does not appear to be associated with transcriptional repression. Rather, genes containing gbM are ubiquitously expressed at moderate to high levels compared with non-gbM genes (4, 5, 9), and within gbM genes there is a correlation between transcript abundance and methylation levels (10, 11).It has been proposed ...

To understand the variation in genomic patterning of DNA methylation we compared methylomes of 34 diverse angiosperm species. By analyzing whole-genome bisulfite sequencing data in a phylogenetic context it becomes clear that there is extensive variation throughout angiosperms in gene body DNA methylation, euchromatic silencing of transposons and repeats, as well as silencing of heterochromatic transposons. The Brassicaceae have reduced CHG methylation levels and also reduced or loss of CG gene body methylation. The Poaceae are characterized by a lack or reduction of heterochromatic CHH methylation and enrichment of CHH methylation in genic regions. Reduced CHH methylation levels are found in clonally propagated species, suggesting that these methods of propagation may alter the epigenomic landscape over time. These results show that DNA methylation patterns are broadly a reflection of the evolutionary and life histories of plant species.

Recent studies have shown that one of the parental subgenomes in ancient polyploids is generally more dominant, having retained more genes and being more highly expressed, a phenomenon termed subgenome dominance. The genomic features that determine how quickly and which subgenome dominates within a newly formed polyploid remain poorly understood. To investigate the rate of emergence of subgenome dominance, we examined gene expression, gene methylation, and transposable element (TE) methylation in a natural, <140-year-old allopolyploid (Mimulus peregrinus), a resynthesized interspecies triploid hybrid (M. robertsii), a resynthesized allopolyploid (M. peregrinus), and progenitor species (M. guttatus and M. luteus). We show that subgenome expression dominance occurs instantly following the hybridization of divergent genomes and significantly increases over generations. Additionally, CHH methylation levels are reduced in regions near genes and within TEs in the first-generation hybrid, intermediate in the resynthesized allopolyploid, and are repatterned differently between the dominant and recessive subgenomes in the natural allopolyploid. Subgenome differences in levels of TE methylation mirror the increase in expression bias observed over the generations following hybridization. These findings provide important insights into genomic and epigenomic shock that occurs following hybridization and polyploid events and may also contribute to uncovering the mechanistic basis of heterosis and subgenome dominance.