5 Live Offspring Produced from Reproductive Material Recovered During the Annual Cull of Bison from Yellowstone National Park

Abstract

Bison from Yellowstone National Park (YNP) have a unique genetic history that makes them a conservation priority for the species. Unfortunately, there is a high prevalence of the zoonotic disease brucellosis in the herd. Part of the management strategy for controlling the disease and herd size in YNP is an annual culling process. This culling may result in loss of valuable genetic material and, until now, has been a missed opportunity to recover and preserve those genetics. The goal of this project was to demonstrate the feasibility of producing healthy offspring from reproductive material collected during the nonbreeding season from bison postmortem and to determine effects of donor parameters, including sexual maturity and pregnancy status on IVF outcomes. Oocytes were collected from ovaries harvested from bison within 2 h of slaughter during winters of 2014 to 2017 for use in in vitro embryo production (IVP). Frozen-thawed semen from 7 YNP bulls collected via electroejaculation in vivo or from epididymal flushes postmortem was used for IVF. Embryos were produced using standard cattle IVP procedures. On Days 7, 7.5, and 8 of in vitro culture, embryos were assessed for developmental stage and quality, and Grade 1 and 2 embryos were vitrified on Cryotops® (Kitazato, Tokyo, Japan) in a two-step equilibration process. Embryos were then stored until the breeding season when they were warmed, cultured for 6 h, evaluated for survival, and transferred to healthy bison recipients. Data were analysed using a Student’s t-test. Age did not affect the mean number of oocytes collected from a mature female (n = 254) v. juvenile (n = 27; 24 ± 16 v. 24 ± 13), embryo cleavage rates (54 ± 24% v. 45 ± 24%), or the rate of blastocyst development (8 ± 12% v. 7 ± 8%, respectively). Pregnancy status did have a significant effect on the number of oocytes collected per female (pregnant 22 ± 15%, n = 258; nonpregnant 26 ± 20%, n = 82; P = 0.04) and the percent of oocytes that cleaved (pregnant 56 ± 23%, n = 189; nonpregnant 47 ± 24, n = 61; P = 0.005). No difference was detected in the percent of embryos that reached the blastocyst stage of development (pregnant 8 ± 10%, nonpregnant 8 ± 15%). In August 2016, 12 embryos were transferred singly or in pairs to 10 recipient bison synchronized using a standard synch protocol. Ten days following removal of the CIDR, embryos were transferred to recipient bison with a palpable corpus luteum. In spring of 2017, a healthy female calf was born to a recipient who had received 2 Grade-2 blastocysts as graded at the end of the 6-h incubation period. The genetic dam of the calf was a pregnant adult bison cow. This calf demonstrates that live offspring can be generated from oocytes collected from bison postmortem in the nonbreeding season. Although cleavage rates were higher with oocytes from pregnant bison, viable embryos were generated from nonpregnant adults and juvenile females, warranting continued collection of their oocytes and banking of resulting embryos. To our knowledge, this is the first bison calf produced by in vitro production using postmortem reproductive material.