1.Chongqing Academy of Animal Sciences, Chongqing 402460, China. 2. Key Laboratory of Pig Industry Science, Ministry of Agriculture, Chongqing 402460. 3. National Center of Technology Innovation for Swine, Chongqing 402460. 4. Chongqing Key Laboratory of Pig Industry Sciences, Chongqing Academy of Animal Sciences, Chongqing 402460. 5. Farm Animal Genetic Resource Exploration and Innovation Key Laboratory of Sichuan Province, Sichuan Agricultural University, Chengdu 611130