Abstract
Under intensive captive conditions, wild-caught flathead grey mullet (Mugil cephalus) females remain arrested at early gonad development and males do not produce sperm. With the aim to induce gametogenesis and obtain fertilized eggs, adult female and male grey mullet were treated with M. cephalus single-chain recombinant gonadotropins (rGths), follicle-stimulating (rFsh) and luteinizing (rLh) hormones. In Experiment 1, a weekly dose of rFsh (15 µg kg−1) significantly (P < 0.001) increased plasma concentration of 17β-estradiol in females and induced oocyte growth up to a maximum diameter of 425 ± 19 µm after 9 weeks of treatment. In males, spermiogenesis (> 75% motile spermatozoa) was induced after 5 weeks to produce low quantities (29.3 ± 6.1 µL) of viscous sperm. However, the application of therapies based on human chorionic gonadotropin (hCG) or gonadotropin releasing hormone agonist (GnRHa) were ineffective to complete oocyte maturation. In Experiment 2, fish were treated with weekly injections of both rFsh and rLh at different doses (from 2.5 to 12 µg kg−1). Oocyte diameter reached 609 ± 5 µm prior to oocyte maturation and ovulation, which was induced with 30 µg kg−1 of rLh and 40 mg kg−1 of progesterone. Higher quantities of good quality sperm (92.7 ± 24.7 µL) were obtained after rLh treatment of males and used to fertilise the eggs. Although the percentage of fertilisation was low (0.4 %), these eggs were able to produce viable larvae (71% hatching rate). In comparison, control groups remained arrested as immature fish. The study demonstrated that both rGths are effective to induce the entire process of gametogenesis in sexually immature male and female grey mullet, which confirmed the central roles of the Gths in teleost gametogenesis. This advance provides the bases of a therapy for the use in the aquaculture of teleosts of commercial interest or the conservation of endangered species.
Competing Interest Statement
IG is associated with the biotech company Rara Avis Biotec, S. L., which produced the recombinant gonadotropins employed in this study. Other than this, there are no patents or products in development to declare.