Effect of breed and embryo sexing on the efficiency of a commercial OPU-ICSI program in mares

Abstract

Breeders of Polo pony mares favour females over males as their sport performance is shown to be superior. This sex preference in breeding Polo horses is a challenge for stud veterinarians as sex sorted semen in horses is not available commercially due to technical difficulties. An alternative is the sexing of embryos. Ovum pick-up (OPU) and ICSI have become popular in show jumping (SJ) horses over the last decade, with the overall efficiency of the process improving steadily, with reported embryo per OPU session of > 1.5 embryos. The in vitro production of embryos not only allows a higher yield of blastocysts compared with in vivo derived embryos, but also their cryopreservation with similar pregnancy results similar to those achieved with fresh embryos. In addition, the possibility of sexing embryos before freezing would be an attractive option to the Polo breeding industry. Although there are several reports on the efficiency of an OPU-ICSI commercial program in SJ, information in Polo mares OPU-ICSI programs is scarce. The objective of this study was to describe the OPU-ICSI outcomes of a commercial program of Polo mares (n=77 cycles) compared to show jumping mares (n=73 cycles), including the likelihood of pregnancy after transferring sexed cryopreserved embryos. A general linear model of variance was created to test the effect of breed (Polo vs. SJ)on antral follicles available for aspiration, recovered oocytes, and number of embryos per OPU-ICSI session. Furthermore, the effect of embryo sexing prior to cryopreservation on pregnancy rate was determined by chi-square test. All OPUs were performed in the UK and oocytes were shipped to an external laboratory in Europe. Pre-implantation gender diagnosis was performed by extended embryo culture and sexing of extruded cells in 53 embryos of Polo mares. Polo mares had a lower (P<0.001) number of punctured follicles and recovered oocytes (14.1±0.6 and 8.2±0.5) compared to SJ mares (19.2±0.7 and 11.0±0.5), respectively. The mean number of embryos per OPU-ICSI session was similar (P>0.1) for SJ and Polo mares (1.38 and 1.41, respectively). Embryos derived from SJ mares (15/22, 68%) were more likely (P<0.05) to develop into a pregnancy than embryos from Polo mares (22/51, 43%). The ongoing pregnancy rate of sexed embryos (6/26, 23.1%) was lower (P=0.016) than that of unsexed embryos (14/25, 56%) from Polo mares, which explains the overall lower pregnancy outcome of Polo compared to SJ mares (unsexed embryos). In conclusion, Polo mares had a lower antral follicle pool than SJ mares, but their oocytes showed similar developmental potential to those of SJ mares. Embryo sexing decreased the chances of having a viable pregnancy after an OPU-ICSI session.