Striking differences in patterns of germline mutation between mice and humans

Summary Recent whole genome sequencing (WGS) studies have estimated that the human germline mutation rate per basepair per generation (∼1.2−10 −8 ) 1,2 is substantially higher than in mice (3.5-5.4−10 −9 ) 3,4 , which has been attributed to more efficient purifying selection due to larger effective population sizes in mice compared to humans. 5,6,7 . In humans, most germline mutations are paternal in origin and the numbers of mutations per offspring increase markedly with paternal age 2,8,9 and more weakly with maternal age 10 . Germline mutations can arise at any stage of the cellular lineage from zygote to gamete, resulting in mutations being represented in different proportion and types of cells, with the earliest embryonic mutations being mosaic in both somatic and germline cells. Here we use WGS of multi-sibling mouse and human pedigrees to show striking differences in germline mutation rate and spectra between the two species, including a dramatic reduction in mutation rate in human spermatogonial stem cell (SSC) divisions, which we hypothesise was driven by selection. The differences we observed between mice and humans result from both biological differences within the same stage of embryogenesis or gametogenesis and species-specific differences in cellular genealogies of the germline.