51 Genome-wide abnormalities resulting from heterogoneic cell division persist in the blastocyst-stage bovine embryo

Mammalian zygotes are expected to cleave into two biparental diploid daughter blastomeres by segregating one copy of each parental genome to each blastomere. Yet, time-lapse monitoring has exposed the frequent incidence of multipolar zygotic division, in which three or four blastomeres are segregated directly. In parallel, genome-wide abnormalities have been encountered in preimplantation bovine, rhesus monkey, and human embryos at different stages of development. Those genome-wide abnormalities are also a key feature of chimeric and/or mixoploid human and bovine viable offspring and human pregnancy complications for which the mechanistic origin remains enigmatic. We have previously demonstrated that whole parental genomes are mis-segregated to distinct cell lineages during the multipolar zygotic division of the bovine embryo, resulting in mixoploid and/or chimeric embryo constitutions. This phenomenon was coined “heterogoneic cell division”. Here we questioned whether genome-wide abnormalities resulting from heterogoneic cell division persist to the blastocyst stage. Therefore, we produced bovine embryos by a standard protocol, including IVM and IVF. Eight to 12 single blastomeres (total of 62) were dissociated from six Day-7 or Day-8 blastocysts developing after tripolar (n = 2) or tetrapolar (n = 4) zygotic division, as identified by time-lapse monitoring. Next, the genome-wide copy number and parental haplotypes of each sample were analysed by a bioinformatics pipeline called haplarithmisis following single-cell whole-genome amplification and SNP genotyping. The subgroup of sampled blastomeres presented a biparental diploid constitution in two blastocysts and a chimeric constitution in four blastocysts. Confirming our previous experiments, heterogoneic cell division coincided in at least four out of six embryos with polyspermic fertilisation. Specifically, three of the chimeric blastocysts contained biparental diploid blastomeres in the compacted embryonic mass and one or two androgenetic blastomeres of a distinct paternal haplotype in the periphery. The fourth contained diandric blastomeres containing two distinct paternal haplotypes in the compacted embryonic mass and one biparental diploid blastomere with complex aneuploidies in the periphery. In conclusion, genome-wide abnormalities generated by heterogoneic cell division in the bovine embryo are recovered at the blastocyst stage. Androgenetic, presumably haploid, blastomeres and blastomeres with complex aneuploidies were often extruded from the compact embryonic mass to the periphery while biparental diploid blastomere lineages seem favourable for further development of the embryo. Yet chimerism and a complete diandric compacted embryonic mass encountered at the blastocyst stage add evidence to the theory that heterogoneic cell division contributes to embryonic arrest and the development of offspring presenting with sex- or imprinting-like disorders, unusual twin types, and complete or partial hydatidiform moles.