Spermatogenesis enhancement in hatchery-produced greater amberjack (Seriola dumerili)

INTRODUCTION Greater amberjack Seriola dumerili (Risso, 1810) is a promising emerging aquaculture species thanks to its rapid growth and consumers’ appreciation. Wild-caught greater amberjack males reared in sea cages showed alteration of plasma sex steroid concentrations, high testicular apoptosis, reduced germ cell proliferation and low sperm quality. We report the effects of gonadotropin releasing hormone agonist (GnRHa) and human chorionic gonadotropin (hCG) administration on testis development and male germ cell proliferation in hatchery-produced greater amberjack. METHODS Four-year-old hatchery-produced greater amberjack males (F1 generation) reared in a sea cage in Salamina (Greece) were treated with GnRHa, either through EVAc implants (50 μg kg-1 body weight) or injections (20 μg kg-1 body weight), hCG (1000 IU kg-1 body weight) or were left untreated as controls. Two fish per group were treated in mid-May, when testes were in active spermatogenesis. Two weeks after treatments, fish were sacrificed and i) gonadosomatic index (GSI) was calculated as 100 × testis weight/body weight; ii) testis samples were fixed in Bouin’s solution and destined to histological analysis and to the immunodetection of the proliferating cell nuclear antigen (PCNA). The effects of the treatments on spermatogonial proliferation and germ cell progression towards meiosis was assessed through the count of the number of PCNA-positive single spermatogonia and the number of spermatocysts (PCNA-positive spermatogonial cysts + spermatocyte cysts). RESULTS & DISCUSSION The treatments resulted in an increase of GSI (untreated: 1.0 ± 0.2; GnRHa implant: 2.0 ± 1.4; GnRHa injection: 2.1 ± 0.8; hCG: 2.6 ± 0.7) and seminiferous tubule diameter (untreated: 142.5 ± 23.1 μm; GnRHa implant: 151.0 ± 14.7 μm; GnRHa injection: 196.1 ± 50.8 μm; hCG = 191.4 ± 8.3 μm). According to the subjective histological evaluation, testes of treated fish showed an increase of germinal epithelium height, larger lumina of seminiferous tubules and more abundant luminal spermatozoa compared with untreated controls. Fish treated with hCG showed the most dramatic changes, characterized by confluence of seminiferous tubules in large sperm masses in the internal testicular region. The hormone treatments resulted in both a decrease of proliferating single spermatogonia (untreated: 108.1 ± 1.3; GnRHa implant: 77.9 ± 47.4; GnRHa injection: 37.3 ± 9.2; hCG: 24.7 ± 21.3 cells/mm2) and an increase of PCNA-positive spermatocysts (untreated: 839.4 ± 12.8; GnRHa implant: 959.2 ± 30.8; GnRHa injection: 868.6 ± 379.5; hCG: 2074.0 ± 38.4 spermatocysts/mm2). In conclusion, all the tree treatments were effective in inducing testicular maturation through the stimulation of germ cell progression towards meiosis. Although the injection of hCG showed the most marked overall effects, GnRHa implantation was still able, after two weeks, to support both spermatogonial proliferation and progression towards meiosis, thus suggesting the capacity to sustain spermatogenesis over a longer period compared with the other two treatments. The project received funding from the ERA-NET Cofund BlueBio program (BESTBROOD project).