Figla gene roles in the proliferation of oocytes in the olive flounder Paralichthys olivaceus

Factor in the germ line alpha (a basic helix-loop-helix transcription factor, FIGLA) gene specifically expresses in female germ cells and is essential for ovarian differentiation and development in vertebrates. However, research on the roles in fish gonadal differentiation and development is limited. In this study, we cloned and characterized a figla gene from the olive flounder, Paralichthys olivaceus, an important marine cultured fish in China, Japan, and Korea. The full-length cDNA of the Po-figla is 1316 bp along with a 609 bp open reading frame. Real-time quantitative polymerase chain reaction analysis (qPCR) revealed that it was abundantly expressed in the ovary and female brain. During stages I-V of the ovary, the level of Po-figla mRNA reached the peak at stage II, and then gradually decreased, while Po-figla mRNA was slightly expressed along with the testicular development. Further, one color and two-color in situ hybridization results showed that the gene was mainly located in oocytes. Knocking down Po-figla in the ovary using RNAi led to its down-regulated expression and the reduction of oocyte number. In addition, qPCR results presented the differential expression patterns of Po-figla during gonadal differentiation period. In the exogenous 17 beta-estradiol treatment group (5 ppm), Po-figla expression levels constitutively increased in the differentiating and differentiated ovaries, while in the testosterone (5 ppm) treatment group, it was lower expressed in the testis during testicular differentiation period. Our findings provide evidence that Po-figla is involved in P. olivaceus ovarian differentiation and development, especially might have roles in the proliferation of fish oocytes. These findings provide the basic data for fish reproduction and gonadal development regulation in aquaculture.