83 Production of live calves after transfer of in vitro-produced embryos in synchronised wood bison (Bison bison athabascae)

Wood bison (Bison bison athabascae) are a threatened subspecies native to Canada. They are culturally significant to the First Nations peoples and a keystone species in the ecosystem they inhabit. As part of an overall effort to develop reproductive technologies to establish a functional germplasm biobank for bison conservation, the objective of the study was to identify factors that influence post-implantation embryo viability. Pregnancy rates after the transfer of in vitro-produced (IVP) embryos (1) vitrified at different stages of development (morula, early blastocyst, blastocyst and expanded blastocyst) and on different days post-fertilisation (7–8 days; Exp. 1), and (2) vitrified, frozen in glycerol or fresh (Exp. 2) were compared. The study was completed in two consecutive years (Exp. 1 and 2). Embryos were produced from wood bison oocytes collected by transvaginal ultrasound-guided follicular aspiration subjected to IVF with frozen-thawed wood bison semen. Female bison between 3 and 16 years of age were used as surrogates in October (n = 28 and 26 in Exp. 1 and 2, respectively). A single embryo was transferred into the uterine horn ipsilateral to the corpus luteum 7 days post-ovulation. Transrectal ultrasonography was done at 30 and 60 days post-ovulation to diagnose pregnancy. Data were analysed by Fisher’s exact test. In Exp. 1, there was no difference among the four embryo stage groups for pregnancy per embryo transferred (P/ET) at 30 days post-ovulation (P = 0.26), but P/ET tended to be greater among morula-stage embryos than blastocyst-stage embryos (3/7 (42.9%) vs. 2/21 (9.5%); P = 0.08). The day the embryo was frozen (i.e. 7 to 9 days post-fertilisation) had no effect on P/ET at 30 or 60 days. At the 60-day pregnancy check, two of the five previously confirmed pregnancies were no longer pregnant. The transfer of morulae resulted in a higher P/ET at 60 days than the other groups combined (3/7 (42.9%) vs. 0/21 (0%); P = 0.01). Three calves were carried to term and were born in July 2020; however, one calf died at birth as a result of dystocia. In Exp. 2, pregnancies were produced after the transfer of either fresh or vitrified embryos, but embryos frozen in glycerol produced no pregnancies. The 30-day pregnancy rate was 4/9 (44.4%) vs. 3/8 (37.5%) vs. 0/9 (0%), respectively (P = 0.20), and the 60-day pregnancy rate was 2/9 (22.2%) vs. 2/8 (25%) vs. 0/9 (0%), respectively (P = 0.50). Three calves were carried to term and were born in July 2021. In conclusion, the vitrification method of cryopreserving embryos resulted in a pregnancy rate similar to that after the transfer of fresh embryos. Results also suggest that cryopreservation at the morula stage will yield the best outcome for the purpose of biobanking bison embryos. Future efforts will focus on in vitro culture conditions to improve embryo development rates.

Research was supported by the Natural Sciences and Engineering Research Council of Canada.