Recombinant follicle-stimulating hormone induces vitellogenesis and spermatogenesis in flathead grey mullet (Mugil cephalus)

Abstract

Introduction. The flathead grey mullet (Mugil cephalus) is a catadromous fish that does not spawn spontaneously in captivity. In Mediterranean aquaculture, few males produce flowing milt and females have dysfunctions at the early stages of vitellogenesis and oocyte maturation and ovulation. Research has focused on and hormonal therapies to address final maturation and spawning, but has been less successful in gametogenesis. Recently, promising results with the use of recombinant gonadotropins in finfish species have induced gametogenesis. Therefore, this study aimed to induce vitellogenesis and spermatogenesis in grey mullet using homologous recombinant follicle-stimulating hormone (mugil-rFSH), produced in Chinese Hamster Ovary (CHO) cells.

Methods: Two groups of 12 immature grey mullet (9♀and 3♂, mean weight 990 ± 212g) received weekly intramuscular injections either of CHO cells medium (control) or mugil-rFSH at a dose of 15 μg.kg-1. Approximately every two weeks blood and gonadal samples (with cannulas or abdominal pressure) were taken for analysis. Ovarian development was evaluated in situ examining the diameter of the most advanced vitellogenic oocytes (n=20) and in samples fixed for histology. The presence of milt was assessed and motility measured. Plasma levels of 11-ketoteststerone and 17β-estradiol (E2) were measured in plasma throughout the experiment. Females with oocytes greater than 350 μm and rFSH-treated males received exogenous luteinizing hormone (LH) or human chorionic gonadotropin (hCG) (Solea senegalensis-rLH, hCG, hCG+Progesterone) in different consecutive weeks depending on the ovarian response. Statistical comparisons between groups were made with the Student's t-test. Results and discussion: Initial significant increases (P<0.05) in oocyte diameter and E2 levels were observed in the r-FSH-treated group. In six weeks, oocytes grew from 96.25±26.31 μm to 306.75±116.81 μm and E2levels increased from 37.90±29.47 pg.mL-1to 267.34±120.78 pg.mL-1. In the control group oocyte diameter and E2levels did not change from 92.62±17.16 μm and 33.86±16.25 pg.mL-1respectively. Administration of just r-FSH induced the development to late-vitellogenic oocytes with a maximum diameter of 450 μm. Small quantities of viscous milt with high motility was obtained in rFSH-treated group, whilst no sperm was obtained in control group. The mugil-rFSH treatment induced gametogenesis. However, Solea senegalensis-rLH, hCG and hCG+Progesterone did not complete oocyte growth, maturation and spermiation. Conclusion: The present study demonstrated the efficacy of mugil-rFSH in inducing E2secretion and vitellogenesis and spermatogenesis, but additional hormonal stimulus seems to be required to complete oocyte growth and maturation