1. College of Animal Science and Veterinary Medicine, Shandong Agricultural University, Tai-an City, P. R. China. Post code: 271018
2. These authors contributed equally to this work.
3. Present address: Reproductive Medicine Centre, Affiliated Hospital of Qingdao Medical University, Yuhuangding Hospital of Yantai, Yantai, Shandong, China. Post code: 264000