Translesion DNA synthesis-driven mutagenesis in very early embryogenesis of fast cleaving embryos

During very early embryogenesis of fast cleaving embryos DNA synthesis is short and surveillance mechanisms preserving genome integrity are inefficient implying the possible generation of mutations. To explore this possibility, we have analyzed mutagenesis in Xenopus laevis and Drosophila melanogaster embryos. We report the occurrence of a nucleus-to-cytoplasmic ratio-sensitive high mutation rate in Xenopus. We also show that mutagenesis is dependent upon the master regulator of translesion DNA synthesis (TLS) Rad18 and alleviated by the mismatch repair system. Unexpectedly, using separation of function mutants, we observed a homology-directed repair contribution of Rad18 in reducing the mutation load. While in Drosophila no Rad18 orthologs could be identified, genetic invalidation of TLS in the pre-blastoderm embryo resulted in reduction of both the hatching rate and Single Nucleotide Variations on specific chromosome regions in adult flies. Altogether these findings indicate that in the very early Xenopus and Drosophila embryos TLS strongly contributes to the high mutation rate. This may constitute a previously unforeseen source of genetic diversity contributing to the polymorphisms of each individual with implications for genome evolution and species adaptation. Overall design: Mutagenesis analysis during very early Xenopus and Drosophila embryogenesis