Total RNA was extracted from gonads using the RNeasy Mini Kit (Qiagen, USA) according to the manufacturers instructions. The quality and concentration of RNA were measured using NanoDrop 1000 spectrophotometer (Thermo). Total RNA obtained from three individuals at the same stage was pooled into one sample in equal quantities. Finally, all of the samples were used for Solexa sequencing. For small RNA library construction and sequencing, the 16-30 nt size range of RNA was purified from 15% polyacrylamide gel electrophoresis. Subsequently, the adapters were ligated to the 5 and 3 termini of small RNAs and used as templates for reverse transcription, followed by PCR amplification for high throughput sequencing using the Illumina Hiseq 2006 platform.