A comparison of humans and baboons suggests germline mutation rates do not track cell divisions

Abstract

In humans, most germline mutations are inherited from the father. This observation is widely interpreted as resulting from the replication errors that accrue during spermatogenesis. If so, the male bias in mutation should be substantially lower in a closely related species with similar rates of spermatogonial stem cell divisions but a shorter mean age of reproduction. To test this hypothesis, we resequenced two 3-4 generation nuclear families (totaling 29 individuals) of olive baboons (Papio anubis), who reproduce at ~10 years of age on average. We inferred sex-specific mutation rates by analyzing the data in parallel with three three-generation human pedigrees (26 individuals). The mutation rate per generation in baboons is 0.55x10-8 per base pair, approximately half that of humans. Strikingly, however, the degree of male mutation bias is approximately 3:1, similar to that of humans; in fact, a similar male bias is seen across mammals that reproduce months, years or decades after birth. These results echo findings in humans that the male bias is stable with parental ages and cast further doubt on the assumption that germline mutations track cell divisions. Our mutation rate estimates for baboons raise a further puzzle in suggesting a divergence time between apes and Old World Monkeys of 67 My, too old to be consistent with the fossil record; reconciling them now requires not only a slowdown of the mutation rate per generation in humans but also in baboons.