Timing of early gonadal differentiation and effects of estradiol-17β treatments on the sex differentiation in Largemouth bass (Micropterus salmoides).

In this study, an efficient estradiol-17β (E)-induced feminization method was established based on the timing of early gonadal differentiation in Largemouth bass (Micropterus salmoides). Histological section results showed that from 20 days post-hatch (dph) to 30 dph, the germ cells gradually differentiated into oogonium and spermatic deferent, respectively. Moreover, female-biased genes Foxl2 and Cyp19a1a were up-regulated to the first peak at 20 dph, while the male-biased genes Dmrt1 were up-regulated to the first peak at 30 dph. These results indicated that the timing of early gonadal differentiation in Largemouth bass was between 20 and 30 dph. Therefore, 15 dph Largemouth bass with a body length of 15.10 ± 0.09 mm were chosen, and four E-treated diets were set as 0 (E0, control), 50 mg/kg E (E50), 100 mg/kg E (E100), and 200 mg/kg E (E200). After feeding with E-treated diets for 60 days, female ratios were 55%, 100%, 100%, and 100% in E0, E50, E100, and E200 groups, respectively. No intersex fish were observed in all the groups. However, 30% of females in the E200 group possessed thinner ovaries, with smaller ovary cavity structures and a decreased number of primary oocyte cells than those in other groups. Besides, the Largemouth bass in the E0 group grew more than those in E50, E100, and E200 groups during the E treatments period (P < 0.05). In conclusion, our study suggested that 50-100 mg/kg E-treated diets could effectively induce the feminization of 15 dph Largemouth bass within 60 days duration time, which provided valuable information for the breeding of the all-male Largemouth bass population.