Ythdf m⁶A Readers Function Redundantly during Zebrafish Development

During the maternal-to-zygotic transition (MZT), multiple mechanisms precisely control massive decay of maternal mRNAs. N⁶-methyladenosine (m⁶A) is known to regulate mRNA decay, yet how this modification promotes maternal transcript degradation remains unclear. Here, we find that m⁶A promotes maternal mRNA deadenylation. Yet, genetic loss of m⁶A readers Ythdf2 and Ythdf3 did not impact global maternal mRNA clearance, zygotic genome activation, or the onset of gastrulation, challenging the view that Ythdf2 alone is critical to developmental timing. We reveal that Ythdf proteins function redundantly during zebrafish oogenesis and development, as double Ythdf2 and Ythdf3 deletion prevented female gonad formation and triple Ythdf mutants were lethal. Finally, we show that the microRNA miR-430 functions additively with methylation to promote degradation of common transcript targets. Together these findings reveal that m⁶A facilitates maternal mRNA deadenylation and that multiple pathways and readers act in concert to mediate these effects of methylation on RNA stability.