Review: Reproductive physiology of bison and application of assisted reproductive technologies to their conservation

Bison are an ecologically and culturally important species on the European and North American continents. Their near extirpation was met with conservation efforts that prevented their extinction but left few animals or highly fragmented populations. Contemporary conservation efforts are focused on building ecologically and genetically sustainable bison herds for long-term conservation of the species. Assisted reproductive technologies (ARTs) can play a key role in building these herds by facilitating the movement of genetics in the form of gametes and embryos, while protecting animal well-being and ensuring biosecurity of existing bison herds. In addition, ARTs such as gamete and embryo cryopreservation can be used to protect against future losses of genetic diversity through biobanking. In this review, a brief summary of basic bison reproductive physiology is presented followed by an overview of the current state of ART in Bison bison (American bison) and Bison bonasus (European bison or wisent). Research on ART ranging from artificial insemination to in vitro embryo transfer and cloning is discussed with particular regard to the application of ART for conservation purposes. While significant progress has been made in ART for bison, there are still many opportunities to improve these technologies and expand their impact for bison conservation.

Strategies for oocyte collection and in vitro embryo production in free-roaming bison herds

The study was conducted to test the feasibility of protocols for field collection of cumulus-oocyte complexes (COC) for in vitro embryo production (IVP) in wild bison. The study was done with captive wood bison during the anovulatory season. In Experiment 1, the efficiency of transvaginal ultrasound-guided COC collection was compared between bison restrained in a squeeze chute without sedation vs in lateral recumbency after chemical immobilization using a dart gun (n = 8/group). In Experiment 2, a 2 × 2 design was used to examine the effects of superstimulation treatment [single dose of equine chorionic gonodotrophin (eCG) vs multiple doses of follicle stimulating hormone (FSH)] and method of drug administration (manual injection vs field darting) on COC collection and IVP. In Experiment 1, no difference was detected between chute-restrained vs chemically immobilized groups in the time required to complete COC collections, the number of follicles aspirated (11.5 ± 1.9 vs 9.3 ± 1.8; P = 0.4) or the COC recovery rate [COC recovered/follicle aspirated; 58/92 (63%) vs 44/69 (64%); P = 0.9]. In Experiment 2, no differences were detected between superstimulation treatments (eCG vs FSH). The total number of follicles available for aspiration did not differ between manual injection and field darting (23.9 ± 2.7 vs 21.6 ± 1.9; P = 0.4). Compared with the random start unstimulated group, the embryo production rate was higher [18/132 (14%) vs 53/189 (28%); P = 0.04] after wave synchronization and superstimulation. Results suggest that COC collection is equally feasible in a recumbent position after chemical immobilization as those bison restrained in a standing position in a hydraulic chute. Ovarian superstimulation with a single-dose eCG protocol is as effective as a multiple-dose FSH protocol, and field darting is as effective as chute-side administration of superstimulation treatments. The strategies in the present study are ready to be incorporated into field collections in free-roaming bison herds.

Cryopreservation of bison semen without exogenous protein in extender and its fertility potential in vitro and in vivo following fixed-time artificial insemination

Successful cryopreservation of bison semen is fundamental for restoration of genetic diversity in Canada's wood bison. Conventional bovine semen extenders contain animal products, such as egg yolk and milk, which are undesirable because of biosecurity risks and undefined composition. In this study, we examined the efficacy of an exogenous protein-free extender containing cholesterol-cyclodextrin complex (CC) to cryopreserve bison semen. The study also provided an opportunity to determine the effectiveness of different ovulation synchronization protocols for fixed-time artificial insemination in bison. Semen was collected from wood bison bulls via electroejaculation and cryopreserved in either Tris-egg yolk-glycerol (called 'TEYG') extender or pretreated with CC (2 mg/mL semen) and diluted in Tris-glycerol (collectively called 'CC-TG') extender. Post-thaw sperm motion characteristics and in vitro fertilization of cattle oocytes confirmed that CC alone without egg yolk protected bison sperm during cryopreservation process. In the first fertility trial, however, no pregnancy was obtained following fixed-time artificial insemination of bison cows with CC-TG extender. In a follow-up trial, low concentration of CC (1 mg/mL semen) resulted in better post-thaw sperm motion characteristics, fertility rate, and birth of live calves following fixed-time artificial insemination. Results showed that 1 mg CC/mL semen completely replaced egg yolk in bison semen extender. In addition, both follicular ablation and steroid treatment protocols provided ovulation synchrony to permit successful application of fixed-time artificial insemination in bison. This is the first report on the birth of live bison calves following fixed-time artificial insemination using semen cryopreserved in a defined extender.

Reproduction in female wild cattle: Influence of seasonality on ARTs

Wild cattle species, often considered less alluring than certain conservation-dependent species, have not attracted the same level of interest as the charismatic megafauna from the general public, private or corporate donors, and other funding agencies. Currently, most wild cattle populations are vulnerable or threatened with extinction. The implementation of reproductive technologies to maintain genetically healthy cattle populations in situ and ex situ has been considered for more than 30 years. Protocols developed for domestic cattle breeds have been used with some success in various wild cattle species. However, inherent differences in the natural life history of these species makes extrapolation of domestic cattle protocols difficult, and in some cases, minimally effective. Reproductive seasonality, driven by either photoperiod or nutritional resource availability, has significant influence on the success of assisted reproductive technologies (ARTs). This review focuses on the physiological processes that differ in breeding (ovulatory) and non-breeding (anovulatory) seasons in female cattle, and the potential methods used to overcome these challenges. Techniques to be discussed within the context of seasonality include: estrus synchronization and ovulation induction, ovarian superstimulation, artificial insemination (AI), multiple ovulation embryo transfer (MOET), and ovum pick-up (OPU) with in vitro fertilization (IVF) and embryo transfer (ET).

Effect of season and superstimulatory treatment on in vivo and in vitro embryo production in wood bison (Bison bison athabascae)

Two experiments were done using a two-by-two design to determine the effects of season and superstimulatory protocol on embryo production in wood bison. In Experiment 1 (in vivo-derived embryos), ovarian superstimulation was induced in female bison during the ovulatory and anovulatory seasons with either two or three doses of FSH given every-other-day (FSH × 2 vs. FSH × 3, respectively). Bison were given hCG to induce ovulation, inseminated 12 and 24 hr after hCG, and embryos were collected 8 days after hCG (n = 10 bison/group). In Experiment 2 (in vitro embryo production), ovarian superstimulation was induced in female bison during the ovulatory and anovulatory seasons with two doses of FSH, and in vivo maturation of the cumulus-oocyte complexes (COC) was induced with hCG at either 48 or 72 hr after the last dose of FSH. COC were collected 34 hr after hCG, and expanded COC were used for in vitro fertilization and culture. In Experiment 1, the number of follicles ≥9 mm, the proportion of follicles that ovulated, the number of CL, and the total number of ova/embryos collected did not differ between seasons or treatment groups, but the number of transferable embryos was greater (p < .05) in the ovulatory season. In Experiment 2, no differences were detected between seasons or treatment groups for any end point. The number of transferable embryos produced per bison was greatest (p < .05) using in vitro fertilization and was unaffected by season (1.5 ± 0.2 and 1.1 ± 0.3 during anovulatory and ovulatory seasons, respectively), in contrast to in vivo embryo production which was affected by season (0.1 ± 0.01 and 0.7 ± 0.2 during anovulatory and ovulatory seasons, respectively). Results demonstrate that transferable embryos can be produced throughout the year in wood bison by both in vivo and in vitro techniques, but the efficiency of embryo production of in vivo-derived embryos is significantly lower during the anovulatory season.

In vitro-production of embryos using immature oocytes collected transvaginally from superstimulated wood bison (Bison bison athabascae)

Two experiments were done to test the hypothesis that morphologic characteristics of wood bison cumulus-oocyte complexes (COC) are reflective of the ability of the oocyte to develop to an advanced embryonic stage after in vitro maturation, fertilization and culture, and to determine the effect of prolonging the interval from the end of superstimulation treatment to oocyte collection (FSH starvation period). Experiments were done during the anovulatory season. In Experiment 1, ovarian superstimulation was induced in 10 bison with two doses of FSH given at 48 h intervals beginning at the time of follicular wave emergence. COC were collected 3 days (72 h) after the last dose of FSH by follicular aspiration and classified as compact, expanded or denuded. The COC were matured in vitro for 24 h before fertilization in vitro (Day 0). Embryo development was assessed on Days 3, 7 and 8. The blastocyst rate was 7/34, 2/10 and 0/3 in COC classified as compact, expanded and denuded, respectively; however, only compact COC resulted in embryos that reached the expanded blastocyst stage. In Experiment 2, COC were collected at either 3 or 4 days (72 or 96 h) after the last dose of FSH (n = 16 bison/group) to determine the effect of the duration of FSH starvation on oocyte competence. The COC were classified as compact good (>3 layers of cumulus cells), compact regular (1-3 layers of cumulus cells), expanded or denuded, and then matured, fertilized and cultured in vitro. Although follicles were larger (P < 0.05) in the 4-day FSH starvation group, there was no effect of starvation period on the distribution of COC morphology; overall, 112/194 (57.7%) were compact, 29/194 (26.3%) were expanded, 39/194 (20.1%) were denuded, and 14/194 (7.2%) were degenerated (P < 0.05). Similarly, there was no effect of starvation period on embryo development. Compact good COC had the highest cleavage (88%) and blastocyst rates (54%; P < 0.05), followed by compact regular COC at 73% and 25%, respectively. Expanded and denuded COC had low cleavage (40% vs. 59%, respectively) and blastocyst rates (5% vs. 8%, respectively). We conclude that morphologic characteristics of wood bison COC are reflective of the ability of the oocyte to develop into an embryo in vitro. Importantly, oocytes collected from superstimulated bison during the anovulatory season were competent to develop to the blastocyst stage following in vitro maturation, fertilization and culture.

In vivo and in vitro maturation of oocytes collected from superstimulated wood bison (Bison bison athabascae) during the anovulatory and ovulatory seasons

Experiments were done to compare the in vivo and in vitro maturational characteristics of cumulus-oocyte complexes (COC) collected from live wood bison. In Experiment 1 (anovulatory season), follicular ablation was done to synchronize follicle wave emergence among bison on Day -1, and FSH was given on Days 0 and 2. Bison were then assigned to 5 groups (n=5/group) in which COC were collected by transvaginal follicle aspiration on Day 4 and either fixed immediately with no maturation (control), matured in vitro for 24 or 30h, or collected on Day 5 after in vivo maturation for 24 or 30h (i.e., after hCG treatment). In Experiment 2 (ovulatory season), bison were treated as described for Experiment 1, but PGF2α (cloprostenol) was given to control the luteal phase on Days -9 and 3. In both experiments, cumulus cell expansion was more extensive following in vivo than in vitro maturation, and the percentage of fully expanded COC was highest in the in vivo 30h groups. Nuclear maturation occurred more rapidly in vitro; 60-70% of oocytes were at the MII stage 24h after in vitro maturation while only 25-27% of oocytes had reached the MII stage after 24h of in vivo maturation. In conclusion, nuclear maturation occurred more rapidly during in vitro vs. in vivo maturation, but was associated with less cumulus expansion than in vivo maturation. In vivo oocyte maturation was more complete at 30 vs. 24h after hCG treatment. Season had no effect on the maturational capacity of wood bison oocytes.