Foxl3, a Target of miR-9, Stimulates Spermatogenesis in Spermatogonia During Natural Sex Change in Monopterus albus

Two foxl2 paralogs, foxl2 (also named foxl2a) and foxl3 (also named foxl2b), were considered as fish-specific duplicates. Both belong to the Forkhead box family of transcription factors, which play important roles in regulating reproduction involved in sexual differentiation and regulation of primordial germ cell and gonadal somatic cell development. We isolated the complete foxl3 cDNA from the rice field eel (Monopterus albus), which undergoes a natural female-to-male sex change via an intersex stage during its life cycle. The deduced amino acid sequence of M albus Foxl3 exhibited high identity with that of the European sea bass (Dicentrarchus labrax, 82.9%). The foxl3 expression levels in gonads were increased during the natural sex change process in M albus. The relative expression level in the testis was greater than 40-fold greater than in the ovary (P < .05). A dual-luciferase assay confirmed that the miR-9, but not the miR-430 family, bound to the foxl3 3' untranslated region of M albus. Foxl3 was primarily expressed in granulosa cells and previtellogenic follicles in the ovary and in spermatogonia and Sertoli cells in the testis. In conclusion, Foxl3 and miR-9 may be involved in physiological processes that promote oocyte degeneration in the ovotestis and stimulating spermatogenesis in spermatogonia in M albus.