Expression profiles of Rspo1, 2 and 3 during the critical period of sex determination/differentiation in the medaka gonads assayed by real-time PCR. Relative mRNA level (copy number of each gene/EF1-α) represents the mean ± S.E. of samples obtained from at least 15 fish at each developmental stages. Mean values with different uppercase, different lowercase and asterisks indicate significant difference of each gene in the female gonads, male gonads, between female and male gonads, respectively (P < 0.05). (▮ = female □ = male; S = stage; dah = day after hatching).