Maternal DNA repair safeguards genome stability during the oocyte-to-embryo transition.

De novo mutations are a major source of genetic variation and disease risk, yet the developmental timing and mechanisms underlying their origin require further investigation. While germ cells have traditionally been considered the primary source of these mutations, increasing evidence suggests that a substantial fraction arise after fertilization. Here, we investigated the role of maternal DNA repair in shaping mutagenesis during this critical window by using a mouse model with oocyte-specific disruption of the homologous recombination factor RAD51 and a combination of cellular and molecular analyses. Loss of maternal RAD51 led to the accumulation of DNA double-strand breaks in oocytes without impairing their growth, meiotic maturation, or fertilization competence. In contrast, embryos derived from RAD51-deficient oocytes exhibited increased DNA damage and developmental delay during early cleavage stages. Whole-genome sequencing revealed a significant increase in de novo variants in offspring, the majority displaying intermediate allele frequencies consistent with post-zygotic mosaic mutations. These findings confirm that maternal DNA repair safeguards genome stability across the oocyte-to-embryo transition and identify early embryogenesis as a major source of de novo mutations, with implications for reproductive biology and the origins of genetic diseases.