Sex differentiation and hormonal feminization in pejerrey Odontesthes bonariensis

Abstract

An investigation was conducted on the histological process of gonadal sex differentiation and the suitability of steroid hormone administration for sex control in pejerrey, Odontesthes bonariensis. Light histological examination of the gonads of larvae and juvenile fish revealed that the primordial gonads were already formed at hatching, but remained sexually undifferentiated until 49 days post-hatch (14.9 mm standard length). Germinal (mitosis and meiosis) and somatic (cluster and outgrowth of somatic cells from the ventral and dorsal edges of the gonads) differentiation began virtually simultaneously in ovaries between 49 and 63 days (18.2 mm). Fusion of the free edges of the outgrowths and formation of the ovarian cavity occurred between 91 and 112days (27.5–37.4 mm). Somatic differentiation (formation of the main sperm duct) began concomitant with intense germ cell mitosis at 98 days (30.4 mm), whereas occasional meiotic activity was observed after 119 days (41.4 mm) in male-type gonads. Only one case of intersexuality was observed, which was characterized by the presence of a small number of diplotene-stage oocytes in an otherwise normally developing testis. This situation was deemed abnormal and did not preclude the characterization of pejerrey as a ‘differentiated’ type of gonochorist teleost. Estradiol-17β administered orally at 20 and 50 mg kg −1 diet from 28 to 105 days (10.9–33.7 mm) produced 100% females. The combined results of histological analysis and hormone administration suggest that the critical period of sex determination in pejerrey reared under gradually rising water temperatures (16–23 °C) occurs between 28 and 49 days post-hatch.