RNAs were extracted from yolk sac tissues of indicated genotypes. The whole yolk sac samples were washed with PBS, and then quick frozen in liquid nitrogen and stored at -80°C for later processing for RNA extraction. Yolk sac samples were homogenized in Trizol (Invitrogen) and processed for RNA extraction according to the manufacturer's instructions. BGI in-house kit (BGI-Shenzhen, China)