Early embryos depend on maternally provided factors for their initial stages of development. This maternal control of development gradually ceases during the maternal-to-zygotic transition (MZT), when maternal messages decay and the zygotic genome gets activated. But what regulates this switch? Here, Anming Meng and co-workers investigate this issue and reveal that the Y-box binding protein Ybx1 acts as a global translational repressor in zebrafish oocytes. By using CRISPR/Cas9 to edit the zebrafish ybx1 gene, they report that maternal but not zygotic ybx1 is essential for embryonic development. A detailed analysis of maternal Ybx1 mutant embryos shows that they exhibit severe defects, including impaired oocyte maturation and egg activation, as well as cleavage and gastrulation defects. Using RNA-seq analyses, the researchers further report that MZT is abolished in Ybx1-depleted embryos; maternal mRNA degradation is impaired while early zygotic genes are downregulated. Finally, the authors show that Ybx1-depleted embryos display elevated global translation, suggesting that Ybx1 functions as a general translational repressor. In line with this, they demonstrate that overexpressed Ybx1 localises to and associates with P-bodies, which are known sites of translational repression. Overall, these findings suggest that maternal Ybx1 safeguards zebrafish oocyte maturation and MZT by repressing global translation and thereby maintaining appropriate protein levels.
