Polyploidy of semi-cloned embryos generated from parthenogenetic haploid embryonic stem cells
In mammals, the fusion of two gametes, an oocyte and a spermatozoon, during fertilization forms a totipotent zygote. There has been no reported case of natural parthenogenesis, in which embryos develop from unfertilized oocytes. The genome and epigenetic information of haploid gametes are crucial for the proper development of embryos. Haploid embryonic stem cells (haESCs) are unique stem cells established from uniparental blastocysts and possess only one set of chromosomes. Previous studies have shown that sperm or oocyte genome can be replaced by haESCs with or without manipulation of genomic imprinting for generation of mice. Recently, these remarkable semi-cloning methods have been applied for screening of key factors of mouse embryonic development. While haESCs have been applied as substitute of gametic genome, the fundamental mechanism how haESCs contribute to the genome of totipotent embryos is unclear. Here, we show the generation of fertile semi-cloned mice by injection of parthenogenetic haESCs (phaESCs) into oocytes after deletions of two differentially methylated regions (DMRs), the IG-DMR and H19-DMR. For characterizing the genome of semi-cloned embryos further we establish ESC lines from semi-cloned blastocysts. We report that polyploid karyotypes are observed frequently in semi-cloned ESCs (scESCs). Our results confirm that mitotically arrested phaESCs provide high efficiency for semi-cloning when the IG-DMR and H19-DMR are deleted. In addition, we highlight the occurrence of polyploidy that needs to be considered for further improvement for development of semi-cloned embryos derived by haESC injection.