Animals and Samples Collection

All animals were raised under the same conditions. We obtained 20-, 75- and 270-day-old clinically healthy Duroc and Chinese Meishan boars from the Fine Farm of Hua Zhong Agricultural University. Tissue samples from testes were frozen in liquid nitrogen immediately after collection and stored at −80 °C prior to RNA extraction. A proportion of the samples from the testes were fixed with 4% formaldehyde (40% Formaldehyde solution: distilled water in a 1:9 ratio) solution for histological comparison. The methods were performed in accordance with the approved guidelines from Huazhong Agricultural University, and scientific, ethical and legal principles of the Hubei Regulations for the Administration of Affairs Concerning Experimental Animals. All experimental protocols were approved by the Ethics Committee of Huazhong Agricultural University.