N 6 -Methyladenosine Profile Dynamics Indicates Regulation of Oyster Development by m 6 A-RNA Epitranscriptomes.

The N-methylation of RNA adenosines (N-methyladenosine, mA) is an important regulator of gene expression with critical implications in vertebrate and insect development. However, the developmental significance of epitranscriptomes in lophotrochozoan organisms remains unknown. Using methylated RNA immunoprecipitation sequencing (MeRIP-seq), we generated transcriptome-wide mA-RNA methylomes covering the whole development of the oyster from oocytes to juveniles. Oyster RNA classes display specific mA signatures, with messenger RNAs (mRNAs) and long non-coding RNAs (lncRNAs) exhibiting distinct profiles and being highly methylated compared to transposon transcripts. Epitranscriptomes are dynamic and correspond to the chronological steps of development (cleavage, gastrulation, organogenesis, and metamorphosis), with a minimal mRNA and lncRNA methylation at the morula stage followed by a global increase. mRNA mA levels are correlated to transcript levels and shifts in methylation profiles correspond to expression kinetics. Differentially methylated transcripts cluster according to embryo-larval stages and bear the corresponding developmental functions (cell division, signal transduction, morphogenesis, and cell differentiation). The mA level of transposon transcripts is also regulated and peaks during the gastrulation. We demonstrate that mA-RNA methylomes are dynamic and associated to gene expression regulation during oyster development. The putative epitranscriptome implication in the cleavage, maternal-to-zygotic transition, and cell differentiation in a lophotrochozoan model brings new insights into the control and evolution of developmental processes.