14 Pregnancies produced after fixed-time artificial insemination using sex-sorted sperm in wood bison

Advanced reproductive techniques are being developed for use in threatened wood bison (Bison bison athabascae). There is a need to produce and use female sex-sorted sperm in conservation herds to increase the proportion of female calves. Sex-sorted sperm technology has been shown to have >90% accuracy in domestic cattle; however, the use of sex-sorted sperm has not yet been reported in bison. Therefore, the objective of this experiment was to determine the efficacy of sex-sorted sperm used in a fixed-time AI (FTAI) protocol in bison. Semen was collected via electroejaculation from wood bison bulls (n = 3) at the Native Hoofstock Centre (NHC; Saskatoon, SK). The ejaculate of the bull with the best semen quality underwent sperm sorting (X chromosome bearing (female)) via flow cytometry, in collaboration with STgenetics, and was cryopreserved. Wood bison females (n = 31) were synchronised in January. On Day 0, transvaginal ablation of follicles ≥5 mm was done and a progesterone-releasing intravaginal device (PRID) was inserted. On Day 6, the PRID was removed and bison were given 500 µg cloprostenol IM. On Day 9, bison were given 2500 IU of human chorionic gonadotrophin (hCG) IM and ranked based on the diameter of the dominant follicle. Bison of similar rank were assigned randomly in replicate to be inseminated with a single unit of either sexed or conventional semen 18 h later (AI = Day 9.75). Transrectal ultrasonography was done on Day 6 and 9 to assess the diameter of the dominant follicle, and on Day 10.5 to confirm ovulation. Pregnancy and fetal sex diagnosis were done by transrectal ultrasonography at 30 and 60 days post-AI, respectively. Statistical analyses were done by ANOVA or chi-squared test. There was no difference in the diameter of the dominant follicle between the sexed and conventional groups on Day 6 (10.0 ± 0.5 mm vs. 9.6 ± 0.4 mm; P = 0.59) or 9 (13.3 ± 0.5 mm vs. 12.9 ± 0.7 mm; P = 0.66). A total of 26/31 (84%) bison ovulated following hCG treatment, and there was no difference between groups (P = 0.57). The five bison that did not ovulate after hCG were removed from pregnancy data. There was no difference in pregnancy per ovulation between the sexed vs. conventional semen groups (2/14 (14%) vs. 3/12 (25%); P = 0.49). At the 30-day pregnancy check, one bison in the sex-sorted group was diagnosed as undergoing early embryonic loss (i.e. small amounts of uterine fluid, disassociated embryonic membranes); however, at ∼150 days post-AI, the same bison was confirmed pregnant by transrectal ultrasonography. In the sex-sorted group, one pregnancy examined at 60 days post-AI was a female fetus; the sex of the other, examined at 150 days, was undiagnosed. All three pregnancies in the conventional group were male fetuses. In summary, sex-sorted sperm can be used to produce sex-selected pregnancies in bison. Results warrant future studies in bison to confirm the effectiveness of sex-sorted sperm in large FTAI trials for producing sex-selected calves.

Funding was provided by STgenetics, Mitacs, Toronto Zoo, and the Natural Sciences and Engineering Research Council of Canada.