A quantitative study of testicular germ cell populations in masculinized neomale common carp (Cyprinus carpio L.)

The main objective of the present study was to investigate the effects of 17α-methyltestosterone treatment upon the testicular germ cells of gynogenetic masculinized neomale common carp (Cyprinus carpio L.) in comparison with diploid carp. Gynogenetic common carp progeny (mean body weight, BW, 2.6 ± 0.3 g; mean total length, 10.4 ± 0.5 cm) were treated for a period of 40 days with 17α-methyltestosterone (MT) at a dose of 100 mg kg−1. The oral administration of MT resulted in 61.5-100% of fish exhibiting male gonads. The masculinized neomales exhibited reduced (P < 0.05) body weight (BW = 22.9 ± 0.8) but significantly increased (P < 0.05) mean testis weight (2.1 ± 0.3) and mean gonadosomatic index (GSI = 9.5 ± 0.2%) in comparison with fish not treated with MT (BW 54.8 ± 1.3; GSI = 0.61%). Furthermore, treatment with MT also resulted in an increased number of fish exhibiting abnormal gonads. However, neomales did not exhibit abnormalities in the development of sperm ducts. MT treatment significantly increased germ cell volume, nuclear diameter, nuclear volume and cyst volume (P < 0.01 in all cases) in MT-treated fish compared to untreated fish. The area occupied by seminiferous tubules, the number of Sertoli cells and germ cells per cyst, and the number of Leydig cells were significantly (P < 0.05) greater in fish treated with MT. The carp neomales exhibited approximately 20-60% more Sertoli cells per cyst (P < 0.05). Leydig cell nuclear volume and Leydig cell individual volume were significantly reduced in MT-treated groups (P < 0.05) compared with untreated groups. In conclusion, our study strongly suggests that the abnormal gonadal structure evident in masculinized neomales could be explained by a combination of MT-induced genetic (homozygosity) and anabolic effects (upon germ and somatic cells).