Providing recombinant gonadotropin-based therapies that induce oogenesis from previtellogenic oocytes to produce viable larvae in a teleost, the flathead grey mullet (Mugil cephalus)

Under intensive captive conditions, wild-caught flathead grey mullet (Mugil cephalus) females remained arrested in previtellogenic stages of gonadal development and no sperm could be obtained from males. With the aim to induce oogenesis from previtellogenesis to oocyte maturation, induce the release of sperm and obtain fertilized eggs, female and male flathead grey mullet were treated with Mugil cephalus single-chain recombinant gonadotropins (rGths), follicle-stimulating (rFsh) and luteinizing (rLh) hormones. In Experiment 1, fish were treated with a weekly dose of rFsh (15 μg kg⁻¹), which in females significantly (P < 0.001) increased plasma concentration of 17β-estradiol and induced vitellogenic oocyte growth up to a maximum mean diameter of 425 ± 19 μm after 9 weeks of treatment. 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, from which final oocyte maturation and ovulation was induced with 30 μg kg⁻¹ of rLh and 40 mg kg⁻¹ of progesterone. Good quality sperm (> 75% motile spermatozoa) was obtained from males in both experiments, and in Exp. 2 the addition of rLh induced the production of higher quantities of sperm that were used to fertilise the eggs. Although fertilisation was low (0.4%), these fertilized eggs with embryo development produced viable larvae (71% hatching). In comparison, control females remained arrested at previtellogenesis and control males did not produce sperm. The study demonstrated that both rGths are effective to induce the process of oogenesis in female flathead grey mullet and to obtain flowing sperm from males, adding more data to confirm the roles of the Gths in teleost gametogenesis. This is the first report, in a teleost species, of the use of rGths (rFsh and rLh) to induce oogenesis from previtellogenesis through to maturation to obtain eggs and larvae. This advance provides the bases for the development of therapies for the use in the aquaculture of teleost of commercial interest or the conservation of endangered species.