Implantation of aromatase inhibitor fadrozole and 17β-estradiol antagonistically affect gonad development in the protandrous barramundi (Lates calcarifer)

Barramundi, Lates calcarifer , are protandrous hermaphrodites, maturing and reproducing first as males before naturally changing sex into females several years later. This male-to-female development provides an excellent model for investigating the genetic mechanism underlying natural sex change in fish, and in aquaculture manipulating this process is key to control reproduction and enhance seedstock production. Exogenous hormones and aromatase inhibitors have been used to obtain control of sex in a range of fish. Fadrozole (FAD) is a non-steroidal inhibitor that reduces biosynthesis of estrogenic hormones, however its use has yet to be tested in barramundi. In this study, 15-month-old male barramundi (2102 ± 126 g) were divided into four treatments and either implanted with i) 17β-estradiol (E2) at 8 mg kg −1 , ii) FAD at 8 mg kg −1 , iii) 8 mg E2 and 8 mg FAD kg −1 together, or iv) implanted with cholesterol pellets (controls). After nine weeks, gonadal phenotype was examined histologically and the expression of sex-related genes were quantified by qPCR. All fish with E2 only implants had sex-changed into females, with significantly higher cyp19a1a and foxl2 expression and lower dmrt1 , cyp11b and esr1 expression than control fish. All fish treated with FAD only remained male, and expression of male genes ( dmrt1 and cyp11b ) were significantly upregulated, whilst the female-biased gene foxl2 was downregulated. Combined FAD and E2 treatment resulted in 42 % of fish remaining male, 42 % becoming female and 16 % that were transitional, with each individuals gene expression patterns reflecting their gonadal phenotype. These results show FAD can impede the feminizing effects of E2 in some individuals, indicating potential for holding barramundi as male for longer before they sex-change. These findings highlight utility of E2 and FAD as tools for modulating sex change in barramundi and inform strategies for improved reproductive control in aquaculture breeding programs. • Fadrozole suppressed the feminizing effect of exogenous Estradiol-2 in 42 % of fish. • Fadrozole upregulated the male-biased gene cyp11b and induced overexpression of dmrt1. • Fadrozole downregulated the female-biased gene foxl2, and maintained low expression of cyp19a1a or esr1. • Fadrozole may prevent barramundi sex change, but fertility must be confirmed.