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TGF-β receptors play important roles in mediating TGF-β signals during gonadal development. To identify the functions of TGF-β receptors, including the type I receptor (activin receptor-like kinase 5, ALK5) and type II receptor (bone morphogenetic protein receptor 2, BMPR2), during the gonadal development of S. prenanti, the full-length cDNA sequences of ALK5 and BMPR2 were isolated and characterized. Their expression patterns in developing gonads and in the gonads of exogenous estradiol (E2) -fed fish were analyzed. The cDNAs of ALK5 and BMPR2 were 1925 bp and 3704 bp in length and encoded 501 and 1070 amino acid residues, respectively. ALK5 and BMPR2 were mostly expressed in gonads, particularly in cortical alveoli stage ovaries and mid-spermatogenic stage testes; however, the overall level of BMPR2 mRNA was higher than that of ALK5 during gonadal development. Furthermore, immunohistochemical signals of ALK5 and BMPR2 were mostly detected at chromatin nucleolar oocytes and perinuclear oocytes in ovaries and at spermatocytes and spermatogonia in testes. Exogenous E2 induces the gonadal expression of ALK5 and BMPR2, and BMPR2 is more responsive to E2 than ALK5. These results suggest that ALK5 and BMPR2 might play a potentially vital role in both folliculogenesis and spermatogenesis in S. prenanti.

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