Graphical Abstract Highlights d Haploid ESCs display PGC-like methylation profiles following in vitro cultivation d Parthenogenetic and androgenetic haploid ESCs show different demethylation dynamics d phESCs carrying 3 deleted imprinted regions support normal growth of bimaternal mice d ahESCs carrying 7 deleted imprinted regions produce live full-term bipaternal mice SUMMARYUnisexual reproduction is widespread among lower vertebrates, but not in mammals. Deletion of the H19 imprinted region in immature oocytes produced bimaternal mice with defective growth; however, bipaternal reproduction has not been previously achieved in mammals. We found that cultured parthenogenetic and androgenetic haploid embryonic stem cells (haESCs) display DNA hypomethylation resembling that of primordial germ cells. Through MII oocyte injection or sperm coinjection with hypomethylated haploid ESCs carrying specific imprinted region deletions, we obtained live bimaternal and bipaternal mice. Deletion of 3 imprinted regions in parthenogenetic haploid ESCs restored normal growth of fertile bimaternal mice, whereas deletion of 7 imprinted regions in androgenetic haploid ESCs enabled production of live bipaternal mice that died shortly after birth. Phenotypic analyses of organ and body size of these mice support the genetic conflict theory of genomic imprinting. Taken together, our results highlight the factors necessary for crossing same-sex reproduction barriers in mammals.

Highlights d Increased SCNT cloning by monoallelic deletion of four H3K27me3-imprinted genes d H3K27me3-imprinted gene deletion normalized body and placental weights in cloned pups d Sfmbt2 deletion is the most effective monoallelic deletion for improving SCNT

Mammalian interspecific hybrids provide unique advantages for mechanistic studies of speciation, gene expression regulation, and X chromosome inactivation (XCI) but are constrained by their limited natural resources. Previous artificially generated mammalian interspecific hybrid cells are usually tetraploids with unstable genomes and limited developmental abilities. Here, we report the generation of mouse-rat allodiploid embryonic stem cells (AdESCs) by fusing haploid ESCs of the two species. The AdESCs have a stable allodiploid genome and are capable of differentiating into all three germ layers and early-stage germ cells. Both the mouse and rat alleles have comparable contributions to the expression of most genes. We have proven AdESCs as a powerful tool to study the mechanisms regulating X chromosome inactivation and to identify X inactivation-escaping genes, as well as to efficiently identify genes regulating phenotypic differences between species. A similar method could be used to create hybrid AdESCs of other distantly related species.