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DNA methylation profiling of Daphnia species.

ABSTRACT: 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.

ORGANISM(S): Daphnia pulex Daphnia magna

PROVIDER: GSE103939 | GEO | 2018/12/11

REPOSITORIES: GEO

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Project description:Background: DNA methylation is one of the main epigenetic mechanisms for the regulation of gene expression in eukaryotes. In the standard model, methylation in gene promoters has received the most attention since it is generally associated with transcriptional silencing. Nevertheless, recent studies in human tissues reveal that methylation of the region downstream of the transcription start site is highly informative of gene expression. Also, in some cell types and specific genes it has been found that methylation of the first intron, a gene feature typically rich in enhancers, is linked with gene expression. However, a genome-wide, tissue-independent, systematic comparative analysis of the relationship between DNA methylation in the first intron and gene expression across vertebrates has not been explored yet. Results: The most important findings of this study are: (1) using different tissues from a modern fish, we show a clear genome-wide, tissue-independent quasi-linear inverse relationship between DNA methylation of the first intron and gene expression. (2) This relationship is conserved across vertebrates, since it is also present in the genomes of a model pufferfish, a model frog and different human tissues. Among the gene features, tissues and species interrogated, the first intron’s negative correlation with the gene expression was most consistent. (3) We identified more tissue-specific differentially methylated regions (tDMRs) in the first intron than in any other gene feature. These tDMRs have positive or negative correlation with gene expression, indicative of distinct mechanisms of tissue-specific regulation. (4) Lastly, we identified CpGs in transcription factor binding motifs, enriched in the first intron, the methylation of which tended to increase with the distance from the first exon–first intron boundary, with a concomitant decrease in gene expression. Conclusions: Our integrative analysis clearly reveals the important and conserved role of the methylation level of the first intron and its inverse association with gene expression regardless of tissue and species. These findings not only contribute to our basic understanding of the epigenetic regulation of gene expression but also identify the first intron as an informative gene feature regarding the relationship between DNA methylation and gene expression where future studies should be focused.

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DNA methylation profiling of Daphnia species.

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