Dietary 17 β‐oestradiol and 17 α‐ethinyloestradiol alter gonadal morphology and gene expression of the two sex‐related genes,dmrt1andcyp19a1a, in juvenile barramundi (Lates calcariferBloch)

This study investigated the effect of 17 β‐oestradiol (E2) and 17 α‐ethinyloestradiol (EE2) on the feminization of barramundi (Lates calcarifer). Fish were fed pellets containing 10 mg E2/kg food, 20 mg E2/kg food, 5 mg EE2/kg food and 10 mg EE2/kg food from 30 to 160 days post hatch (dph), which covers the period of initial male gonad differentiation. The effect of E2 and EE2 treatment on gonadal morphology and dmrt1 and cyp19a1a gene expression was analysed by histology and RT‐qPCR. Orally delivered E2 and EE2 induced observable changes in gonad morphology. Previtellogenic oocytes (PO) were observed in 33% and 50% of the fish fed with 20 mg E2/kg at 160 dph and 12 months post hatch (mph), respectively, while PO were not observed in control fish. 10 mg E2/kg treatment did not induce feminization, but significantly suppressed testicular development. Treatment with EE2 resulted in fibrosis within gonad tissues at a dose‐dependent rate. E2 administration resulted in upregulation of the gene cyp19a1a and downregulation of dmrt1. EE2 significantly suppressed expression of dmrt1 at 160 dph and 12 mph, while cyp19a1a was not significantly different at 160 dph and was significantly downregulated at 12 mph. The result of this study showed that E2 is a more suitable hormone than EE2 to induce precocious feminization of barramundi, and when delivered through the feed it has a dose‐dependent feminizing effect.