Frequency of mosaicism points towards mutation-prone early cleavage cell divisions in cattle.

It has recently become possible to directly estimate the germ-line de novo mutation (dnm) rate by sequencing the whole genome of father-mother-offspring trios, and this has been conducted in human1–5, chimpanzee6, mice7, birds8 and fish9. In these studies dnm’s are typically defined as variants that are heterozygous in the offspring while being absent in both parents. They are assumed to have occurred in the germ-line of one of the parents and to have been transmitted to the offspring via the sperm cell or oocyte. This definition assumes that detectable mosaicism in the parent in which the mutation occurred is negligible. However, instances of detectable mosaicism or premeiotic clusters are well documented in humans and other organisms, including ruminants10–12. We herein take advantage of cattle pedigrees to show that as much as ∼30% to ∼50% of dnm’s present in a gamete may occur during the early cleavage cell divisions in males and females, respectively, resulting in frequent detectable mosaicism and a high rate of sharing of multiple dnm’s between siblings. This should be taken into account to accurately estimate the mutation rate in cattle and other species.