170 IN VITRO PRODUCTION OF BISON EMBRYOS

Abstract

The North American bison is a symbol of Native American heritage and the American West as well as being an increasingly important agricultural species. Reproductive technologies in bison lag behind those of cattle. Blastocyst production rates were low (<10%) in the only study of in vitro production of bison embryos (Thundathil et al. 2007 Theriogenology 68, 93–97). Our aims were to assess the application of our bovine in vitro embryo production system (De La Torre-Sanchez 2006 Reprod. Fertil. Dev. 18, 585–596) to bison gametes and evaluate the effect of adding FCS at different stages of the process. Initially, we performed homologous and heterologous inseminations with bison and cattle oocytes collected from abattoir ovaries and frozen-thawed epididymal bison sperm and ejaculated cattle sperm. Ovaries from both species were processed on the same day and a single straw of semen per species was used to fertilize all oocytes in each of 3 to 5 replicates. Culture media and conditions were identical for each treatment. Cleavage rates from the homologous IVF were 87% (86/99) for bison and 82% (164/199) for cattle. Bison oocytes with cattle sperm resulted in 88% (76/86) cleaved and cattle oocytes with bison sperm resulted in 69% cleaved (133/192; P < 0.01). Day 7 blastocyst rates per oocyte and per 8-cell stage, respectively, were 16 and 27% for cattle embryos, 6 and 13% for cattle oocytes with bison sperm, 10 and 16% for bison oocytes with cattle sperm and 7 and 9% for bison embryos. Experiments for bison embryos were then designed to evaluate the effect of adding 10% FCS to the maturation medium and 5, 2.5, or 0% to CDM2, the culture medium used after the 8-cell stage. Cleavage rates for oocytes matured in 10% FCS were 50% (202/405) and were 61% (206/337) in the absence of FCS. Day 7 blastocyst rates per oocyte and per 8-cell stage, respectively, of embryos matured in 10% FCS and then cultured in CDM2 + FCS were 12 and 26% in 5% FCS; 5 and 15% in 2.5% FCS; and 1 and 2.5% in 0% FCS. Similarly, Day 7 blastocyst rates of embryos matured without FCS but in CDM2 + FCS were 13 and 30% in 5% FCS, 1 and 4% in 2.5% FCS and 2 and 6% in 0% FCS. In a subsequent experiment, 5% FCS was added to CDM1 (culture medium for presumed zygotes through the 8-cell stage), CDM2, or both, but not to the maturation medium. Cleavage rates of bison embryos cultured with or without FCS in CDM1 were 63% (62/99) and 72% (145/202), respectively. Blastocyst rates of embryos cultured in CDM1 + FCS were 16% per oocyte and 36% per 8-cell stages in CDM2 + FCS and 0% in CDM2-FCS. Blastocyst rates of embryos cultured in CDM1 without FCS were 16% per oocyte and 25% per 8-cell stage in CDM2 + FCS and were 1% per oocyte and 2% per 8-cell stage in CDM2-FCS (P < 0.10). Adding 5% FCS to culture medium after embryos have reached the 8-cell stage greatly improved blastocyst rates of in vitro-produced bison embryos. Bison reproduction is highly seasonal and this work was conducted outside the bison breeding season. It is unknown if FCS would influence blastocyst rates when oocytes are collected from bison ovaries during the breeding season.