PSV-12 Fertility following germline transplantation in sterile NANOS2 knockout surrogate bulls.

Abstract Due to the nature of traditional beef cattle production systems and the logistical need for natural mating schemes, genetic improvement is largely limited by geographic location. Through spermatogonial stem cell transplantation (SSCT) of germline ablated recipient males, generation of surrogate sire bulls that produce sperm containing the genetics of higher merit males would be possible and allow for improved dissemination of trait-driving genetics through natural breeding. Previously, we used CRISPR-Cas9 gene editing to generate male mice, pigs, and goats with inactivation of the evolutionarily conserved gene NANOS2 and found that the resulting sterility from the ablation of endogenous germ cells created an ideal host for donor-derived sperm production following SSCT. Here, two germline ablated Angus crossbred bulls were generated by CRISPR-Cas9 editing of the NANOS2 gene and transplanted during early pre-pubertal development with spermatogenic stem cells from a Holstein donor male. At maturity, one of the bulls was found to be producing ejaculates with sperm concentrations, motility, and morphology parameters in the range of normal bulls of similar breeds. Post-thaw survival of cryopreserved sperm was also in the normal range and use for in vitro fertilization resulted in embryo production. Genotyping analysis of the embryos indicated that the sperm were donor derived. In addition, pregnancies were generated following natural mating of the surrogate bull. Upon histological analysis of the testes, spermatogenesis was identified in the seminiferous tubules, further establishing the ability of the NANOS2 knockout male to harbor and maintain spermatogenesis following SSCT. In the second bull, limited sperm production occurred after SSCT and although fertility was not achieved, subsequent processing of the testes post-castration revealed sperm in the epididymis therefore suggesting some success in establishing donor-derived colonies of spermatogenesis in the seminiferous tubules. Together, these findings significantly advance surrogate sires’ development as a potential breeding tool for the beef cattle industry to achieve large scale and widespread dissemination of select genetics to accelerate trait improvements.