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The methylome of a cartilaginous fish, Callorhinchus milii, reveals conservation of epigenetic regulation across jawed vertebrates

ABSTRACT: Background Methylation of CG dinucleotides constitutes a critical system of epigenetic memory in bony vertebrates, where it modulates gene expression and suppresses transposon activity. The genomes of studied vertebrates are pervasively hypermethylated, with the exception of regulatory elements such as transcription start sites (TSSs), where the presence of methylation is associated with gene silencing. This system is not found in the sparsely methylated genomes of invertebrates, and establishing how it arose during early vertebrate evolution is impeded by a paucity of epigenetic data from basal vertebrates. Methods We perform whole-genome bisulfite sequencing to generate the first genome-wide methylation profiles of a cartilaginous fish, the elephant shark Callorhinchus milii. Employing these to determine the elephant shark methylome structure and its relationship with expression, we compare this with higher vertebrates and an invertebrate chordate using published methylation and transcriptome data. Results Like higher vertebrates, the majority of elephant shark CG sites are highly methylated, and methylation is abundant across the genome rather than patterned in the mosaic configuration of invertebrates. This global hypermethylation includes transposable elements and the bodies of genes at all expression levels. Significantly, we document an inverse relationship between TSS methylation and expression in the elephant shark, supporting the presence of the repressive regulatory architecture shared by higher vertebrates. Conclusions Our demonstration that methylation patterns in a cartilaginous fish are characteristic of higher vertebrates imply the conservation of this epigenetic modification system across jawed vertebrates separated by 465 million years of evolution. In addition, these findings position the elephant shark as a valuable model to explore the evolutionary history and function of vertebrate methylation.

ORGANISM(S): Callorhinchus milii

PROVIDER: GSE96683 | GEO | 2017/03/24

SECONDARY ACCESSION(S): PRJNA379367

REPOSITORIES: GEO

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Project description:To profile the Daphnia species methylome and to achieve a better understanding of the level of variations in the methylome of Daphnia species, we performed whole genome bisulfite sequencing (WGBSeq) of adult Daphnia magna Bham2 strain and Daphnia pulex Eloise Butler strain (EB45 and EB31 strains). We also analysed the correlation between gene expression and methylation in the two species, using data generated in this study and RNA-seq data from Orsini, et al. 2016. We found that methylation percentage across the genome of Daphnia spp. follows a bimodal distribution. Furthermore, CpG methylation in Daphnia predominantly occurs at coding regions. Although methylation levels significantly decrease towards the 3’ end of a gene with a significant drop in methylation levels from one exon to the neighbouring intron, there is a clear spike in relative methylation levels between exon and intron boundaries, which may be linked to regulation of splicing. We further demonstrate that DNA methylation in Daphnia is responsive to intrinsic and extrinsic factors. We also compared the methylation and gene expression correlations found in Daphnia to publicly available dataset from two other invertebrate species (Apis mellifera and Nasonia vitripennis) and two vertebrate species (Homo sapiens and Mus musculus). We observed that similar to other invertebrates, Daphnia’s genome is sparsely methylated at a lower level and the methylation is predominantly focused at gene body while in vertebrate species the genome is heavily methylated (global methylation). Although the level and distribution of methylation across CpG sites is different between vertebrates and invertebrates it is possible that methylation density at coding regions has the same function between vertebrates and invertebrates. We demonstrate evolutionary conservation of a positive correlation between high methylation density at coding regions and gene expression across vertebrates and invertebrates, leading to potentially ensuring continuous high expression of genes required throughout the life in both vertebrates and invertebrates.

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The methylome of a cartilaginous fish, Callorhinchus milii, reveals conservation of epigenetic regulation across jawed vertebrates

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