Developmental competence of embryos derived from reciprocal in vitro fertilization between Yak (Bos grunniens) and cattle (Bos taurus).

The purpose of this study was to investigate fertilization ability and embryo development to the blastocyst stage after reciprocal in vitro fertilization (IVF) between yak and cattle in an attempt to clarify the problem of low conception rate after mating yak females with cattle bulls. In vitro-matured (IVM) cattle and yak oocytes were inseminated with either Holstein or yak spermatozoa, and after an 18-h of coincubation period, a proportion of the oocytes was fixed and examined for sperm penetration, polyspermy and male pronuclear formation. The remaining oocytes were cultured in vitro and evaluated for cleavage and blastocyst formation rates. The percentage of IVM oocytes penetrated by spermatozoa ranged from 78.5 to 90.5%, and the formation of one or two pronuclei and the incidence of polyspermy did not differ among the different combinations. The cleavage and blastocyst rates were not affected by the species of the sperm, but they were affected by the species of the oocytes (P<0.05), with cattle oocytes having a higher (P<0.05) cleavage and blastocyst rates (69.9 and 31.3%) than yak oocytes (62.7 and 11.5%). The blastocyst formation rate was calculated from the cleaved zygotes. The interaction between sire and oocytes species (P<0.05) influenced blastocyst formation rate, with the highest blastocyst rate occurring in cattle oocytes fertilized with yak spermatozoa (36.5%) and the lowest rate occurring in yak oocytes fertilized with yak spermatozoa (9.4%). The effect of heterosis was apparent at the blastocyst stage, but there was a large reciprocal difference in blastocyst production between crosses. It was concluded that the low conception rate that results from crossing yaks with cattle is not due to either a species-specific block of fertilization or the developmental competence of the early stage embryo.