| Crossbreeding yak (Bos grunniens) with cattle (Bos taurus) is advocated as a means of increasing milk and meat output from the mountainous regions, i.e., their first cross progenies (F1) have significant heterosis, however, the conception rate for crossbreeding is lower. The objectives of this study were to investigate mechanism for the lower conception rates in crossbreeding and to compare the difference in development of embryos from different fertilization combinations (cattle♀×cattle♂, cattle♀×yak♂, yak♀×yak♂and yak♀×cattle♂). The cumulus-oocytes-complexes (COCs) of yaks and cattle which collected from slaughter house were in vitro fertilized either yak sperm or cattle. The fertilization rates, the rates of cleavage and blastocyst in the four different fertilization combinations were observed.In experiment 1, comparison of fertilization in four different fertilization combinations. COCs were aspirated from both yak and cattle ovaries, then, they were washed and placed in 600μl droplets of bicarbonate - buffered TCM199 supplemented with 20% (v:v) FCS, 5μg/ml FSH, 5μg/ml LH and 1μg/ml 17β- estradiol (maturation media) for a period of 24 h incubation. In vitro-matured oocytes were inseminated with either Holstein bull or yak spermatozoa. The oocytes (n=339) were fixed after 18 h coincubation period, and then stained with 1% orcein in 45% acetic acid. The sperm penetration were examined with the number of the pronuclears (PN). 0 PN were classified as unfertilized eggs, 1 PN as abnormal development eggs, 2 PN as normal fertilized egg, >2 PN fertilized eggs as polyspermy. The results showed that the total fertilization rates for cattle♀×cattle♂, cattle♀×yak♂, yak♀×yak♂and yak♀×cattle♂were 84.3%, 90.5%, 82.3%, and 78.5%, respectively. It was higher (P <0.05) for cattle♀×yak♂than yak♀×cattle♂, but there was no significant difference between other fertilization combinations (P>0.05). Correspondingly, their normal fertilization rates were 71.1%, 76.2%, 65.8% and 68.8%; polyspermy rates were 12.0%, 14.3%, 15.2% and 8.6%, respectively. Cattle oocytes with higher total fertilization rates and normal fertilization rates than the yak oocytes, and the yak sperm groups with higher polyspermy rates than the cattle sperm groups. In experiment 2, comparison of fertilization in four different fertilization combinations. The embryos (n=1649) after in vitro fertilization were cultured in IVC which were synthetic oviduct fluid (SOF) medium supplemented with 6μg/ml BSA, 0.5μg/ml myoinositol. The cleavage rates for four groups were evaluated at 48 h post insemination (hpi), and developmental stages of 2-cell, 4-cell, 8-cell ratio were recorded. The results indicated that the groups from cattle oocytes with higher cleavage rates (67.0% for cattle♀×cattle♂, and 72.4% for cattle♀×yak♂) than the groups from yak oocytes (65.0% for yak♀×yak♂and 61.1% for yak♀×cattle♂). It appears that the groups from cattle oocytes with a faster embryonic development than the groups from yak oocytes. In experiment 3, comparison of blastocyst rates in four different fertilization combinations. The culture medium was replaced once at 96 h post insemination and blastocysts were collected on Day 7, 8, 9 and 10 post insemination. The results showed that the total blastocyst rates for cattle♀×cattle♂, cattle♀×yak♂, yak♀×yak♂and yak♀×cattle♂were 29.4%, 37.9%, 11.4%, and 16.3% respectively. Cattle♀×cattle♂, cattle♀×yak♂harvested the blastocyst mainly on Day 7 and 8 (68.1% and 80.7% respectively), but yak♀×yak♂and yak♀×cattle♂mainly on Day 8 and 9 (59.6% and 79.8% respectively). The groups from cattle oocytes with higher blastocyst rate than the groups from yak oocytes, and the groups from cattle oocytes with a faster blastocyst development than the groups from yak oocytes.In conclusion, yak are able to crossbreed with Bos taurus, but the rates of cleavage and blastocyst development of resultant hybrid embryos were different between different fertilization combinations.
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