New insights into the role of myostatin in fish fertility based on the findings in mstnb-deficient Nile tilapia (Oreochromis niloticus)

Myostatin (Mstn) was reported to be involved in multiple biological processes including the regulation of skeletal muscle growth, lipid metabolism and reproduction in vertebrates, therefore, it might play an essential role in the fine balance between growth and reproduction. In order to clarify the roles of Mstn in fish reproduction, we systematically investigated the gonadal development and fertility of mstnb mutants in Nile tilapia (Orochromis niloticus). In the present study, mstnb was found to be expressed in the different cell types of gonads by fluorescence in situ hybridization (FISH). Mutation of mstnb resulted in excessive proliferation of phase II follicles in the ovaries while the numbers of phase III follicles were significantly reduced at 210 days after hatching (dah). Interestingly, significant decline of cyp19a1a gene expression, the serum 17β-estradiol (E2), vitellogenin (VTG) were detected at 210 dah. At 360 dah, the ovarian development of mstnb-/- XX tilapia was partially recovered and female fish spawn regularly, however, they were subfertile and produced the poor-quality eggs with significantly lower offspring survival rate. The dramatic up-regulation of cyp19a1a gene expression and serum E2 level at 360 dah might account for the recovery of ovarian development. In mstnb-/- males, excessive proliferation of spermatogonia and retardation of testicular differentiation were found throughout the whole development stages. In mstnb-/- males, evident declines of gonadosomatic index (GSI), cyp11b2 gene expression, and 11-Ketotestosterone (11-KT) were detected at 210 dah, while the mRNA level of cyp11b2 and serum 11-KT level were significantly upregulated at 360 dah. However, mstnb-/- males were infertile since mutation of mstnb gene led to excessive increase of apoptosis of testicular cells and spermiation obstacle. Taken together, these results indicated that Mstn might be an essential regulator for the maintenance of fish fertility.