m 6 A modulates haematopoietic stem and progenitor cell specification.

N6 -methyladenosine (m6 A) has been identified as the most abundant modification on eukaryote messenger RNA (mRNA). Although the rapid development of high-throughput sequencing technologies has enabled insight into the biological functions of m6 A modification, the function of m6 A during vertebrate embryogenesis remains poorly understood. Here we show that m6 A determines cell fate during the endothelial-to-haematopoietic transition (EHT) to specify the earliest haematopoietic stem/progenitor cells (HSPCs) during zebrafish embryogenesis. m6 A-specific methylated RNA immunoprecipitation combined with high-throughput sequencing (MeRIP-seq) and m6 A individual-nucleotide-resolution cross-linking and immunoprecipitation with sequencing (miCLIP-seq) analyses reveal conserved features on zebrafish m6 A methylome and preferential distribution of m6 A peaks near the stop codon with a consensus RRACH motif. In mettl3-deficient embryos, levels of m6 A are significantly decreased and emergence of HSPCs is blocked. Mechanistically, we identify that the delayed YTHDF2-mediated mRNA decay of the arterial endothelial genes notch1a and rhoca contributes to this deleterious effect. The continuous activation of Notch signalling in arterial endothelial cells of mettl3-deficient embryos blocks EHT, thereby repressing the generation of the earliest HSPCs. Furthermore, knockdown of Mettl3 in mice confers a similar phenotype. Collectively, our findings demonstrate the critical function of m6 A modification in the fate determination of HSPCs during vertebrate embryogenesis.