The incidence of aneuploidy and mosaicism in 2,045 genotyped cattle blastocysts

Chromosomal abnormalities are the most common cause of developmental arrest in mammalian embryos. They can be present consistently in all cells of the embryo or occur as admixtures of karyotypically distinct lineages (mosaics). The estimated incidence of mosaicism ranges from 14% to 82% in human embryo biopsies at the blastocyst stage. In cattle, mosaicism is not well described at a whole-genome level, with findings limited to sex chromosomes. Here, we conducted a retrospective analysis of published data spanning three studies from our lab to establish the incidence and nature of mosaicism in 2,045 bovine blastocysts genotyped using SNP-based approaches. We classified mosaic embryos as those where the inner cell mass and trophectoderm differed in ploidy and/or where embryos had a percentage of cells with aneuploidy ranging from 20% to 80%. We report an aneuploidy incidence of 15.2% (311/2,045), with 25.6% of the aneuploid embryos (80/311) being mosaic. Mosaicism was particularly common (87.5%, n = 7/8) in embryos affected by multiple types of chromosomal errors and in embryos affected only by segmental aneuploidies (50.0%, n = 9/18). The chromosomal abnormalities with the highest incidence of mosaicism were segmental aneuploidies (48.1%, 13/27). Most errors leading to mosaicism occurred post- zygotically (59.4%, 19/51), followed by paternal errors (44.9%, 22/49). Our results reveal an incidence of mosaicism in bovine embryos similar to that of human embryos. Additionally, we demonstrate that ploidy and mosaicism screening can be performed in embryos using the same SNP genotyping data obtained to calculate genomic estimated breeding values.