Expression levels of all unigenes were calculated and then compared between the two tissue samples using the fragments per kilobase of transcript per million reads of library method (FPKM). The false discovery rate (FDR) was adopted to determine the threshold of P-values in multiple tests. DEGs were also annotated with GO assignments, COG assignments and KEGG pathways. The criteria FDR < 0.05 was used to identify DEGs and acted as a threshold of significant difference of gene expression in GO terms, COG classification and KEGG annotation.