Effects of an androgenic growth promoter 17β-trenbolone on masculinization of Mosquitofish (Gambusia affinis affinis)

Endocrine disrupting chemicals can affect normal hormone dependent processes through numerous mechanisms, including ligand mimicky. 17β-Trenbolone (TB), a pharmaceutical, androgenic, anabolic steroid, is a potent agonist of androgen receptors, and has been extensively used as a growth promoter for beef cattle in the US. The effects of TB on adult and newborn mosquitofish (Gambusia affinis affinis) were examined. Two forms of mosquitofish androgen receptor (AR), ARα and ARβ, were cloned. The mRNA expression levels of ARα and ARβ were transiently increased in the anal fin of adult females at day 3 following exposure to TB (1-10 μg/L) or methyltestosterone (MT) (0.1-10 μg/L), a pharmaceutical androgen used as a positive control. Gonopodium differentiation from the adult female anal fin was induced after 28 days of exposure to TB (1-10 μg/L) or MT (0.1-10 μg/L). Gonopodium differentiation also was induced in all mosquitofish fry exposed for 28 days to 0.3, 1 or 10 μg/L TB. Furthermore, spermatozoa were observed histologically in the testes of male fry exposed for 28 days to 1 or 10 μg/L TB; spermatozoa are normally observed only in the testes of mature males. Surprisingly, all female fry exposed for 28 days to 1 or 10 μg/L TB displayed the formation of an ovotestis, as spermatozoa were found in the ovary. Thus, TB, like MT, induced masculinization of the anal fin accompanied by a transient up-regulation of ARα and ARβ in adult females. TB also induced differentiation of the anal fin into a gonopodium in fry of both sexes, stimulated precocious spermatogenesis in the testes of males and the formation of ovotestes in females.