Characterization of gonadotropin-releasing hormone and gonadotropin in jack mackerel (Trachurus japonicus): Comparative gene expression analysis with respect to reproductive dysfunction in captive and wild fish

To better understand the physiological mechanisms underlying reproductive dysfunction, such as impaired vitellogenesis and final oocyte maturation, we assessed endocrinological differences between captive and wild female jack mackerel (Trachurus japonicus). The presence of three gonadotropin-releasing hormone (GnRH) peptides was immunologically evaluated in jack mackerel brain tissues. Full-length cDNAs encoding GnRHs (gnrh1, gnrh2, and gnrh3) and gonadotropin subunits (fshb, lhb, and gpa) were cloned, sequenced, and quantitatively assayed. Of the captive females, 60% failed to undergo vitellogenesis, displaying immature (IM) or atretic oocytes, whereas 80% of wild females were captured during late vitellogenesis (LV) or ovulation (OV). The gnrh1 expression was significantly lower in captive fish than in wild LV and OV fish, while there were no significant differences in the expression of gnrh2 or gnrh3. The expression of fshb was lower in captive IM fish than in wild LV fish, but no significant differences were observed between the captive IM and LV individuals. The lhb expression was elevated in the wild LV and OV fish, and gpa expression was greatest in the wild OV fish. Serum estradiol-17β levels were significantly lower in captive IM fish than in captive LV fish. The results indicate that captive-rearing stress may impair vitellogenesis and negatively influence the transcription of gnrh1 in the brain and GtH synthesis in the pituitary.