RNA isolation and real-time RT–PCR
Total RNA from oocytes was extracted using the RNeasy Mini kit (Qiagen), followed by RT using Superscript RT kit (Bio-Rad). Quantitative RT-PCR was performed using a Power SYBR Green PCR Master Mix (Applied Biosystems, Life Technologies). Relative mRNA levels were calculated by normalizing to the levels of endogenous GAPDH mRNA (internal control). The primers used are listed in Supplementary Table 1.