Male germline vulnerability to DNA damage causes sex-biased mutation

In many vertebrates the male germline mutation rate is higher than that of females, so-called male mutation bias, the causes of which are central to numerous molecular evolution debates including the relationship between life history traits and the mutation rate and the cause of the generation time effect. The male bias was classically considered a consequence of the higher number of cell divisions in the male germline, but recently multiple lines of evidence find this to be an inadequate explanation. The revived damage-induced hypothesis posits that males are either more prone to germline damage or less likely to repair damage, leading to more mutations. The model predicts that, when males and females are treated with approximately equal levels of mutagenic stress, the male germline will accumulate more mutations. Here we provide the first test of the damage hypothesis by exposing male and female mice to mutagenic chemicals with both untreated and a non-mutagenic (PBS) controls, assaying de novo mutations via parent-offspring whole-genome sequencing. As predicted, in the instances where the mutagens caused a net increase to the mutation rate, it went up more in male germline. Comparison with established mutational signatures reveals differential involvement of the base excision repair (BER) pathway. Similarly, transcriptome analysis finds higher expression levels of DNA repair genes in ovary compared to testis for those genes in the BER pathway, a difference that becomes more extreme on administration of mutagens. As parental age, hence number of germline cell divisions, is controlled, we conclude that the male mutation bias is, at least in part, owing to differential damage repair with lower base excision repair in males playing a significant role.