Brief treatment with an aromatase inhibitor during sex differentiation causes chromosomally female salmon to develop as normal, functional males

Abstract

Although many studies implicate sex steroids in the process of sexual differentiation, the exact role played by these substances in lower vertebrates, especially teleost fish, is not clear, since it is not conclusively known whether sex steroids are the cause or an early consequence of sex differentiation. By hormonal manipulation and specific crossings, it is possible to produce allfemale stocks of the otherwise gonochoristic chinook salmon (Oncorhynchus tshawytscha). These allfemale stocks are an excellent model to study the effects of steroids on sexual differentiation since the genetic sex is known before the actual differentiation of the embryonic gonads takes place. In the present study, we show that treatment with a nonsteroidal inhibitor of aromatase, the enzyme that catalyzes the conversion from androgens to estrogens, for only 2 hours when the gonads were bipotent, caused genetic females to develop into normal males. Furthermore, these males had testes which were indistinguishable both in size and in structure from those of genetic males, and completed all the stages of spermatogenesis. At 2 years of age, these males produced viable sperm capable of inducing normal embryonic development when used to fertilize eggs, with a resulting all-female progeny These results provide strong support for Ymamoto's theory (Yamamoto, '69) that androgens and estrogens are natural sex inducers in gonochoristic fish, and suggest that aromatase plays a pivotal role in the sex differentiation of salmon. Thus, by brief treatment with an aromatase inhibitor at a specific time in development, an organism was induced to develop a funtional, phenotypic sex different from its genetic sex.

Exogenous application of estradiol to eggs unexpectedly induces male development in two turtle species with temperature-dependent sex determination

“…In addition, Fadrozole has previously been shown to induce male development by blocking the aromatization of androgens to estrogens in many taxa (Piferrer et al, 1994;Wibbels and Crews, 1994;Chardard and Dournon, 1999;Warner et al, 2005). Intriguingly, our results in two turtle species across four independent experiments are inconsistent with 13 these well-established findings; the application of Fadrozole had no impact on offspring sex ratios in the present study, and applying estradiol to eggs produced nearly all male offspring, a pattern opposite of expectations.…”

“…The lack of an effect of Fadrozole on offspring sex ratios is difficult to explain considering the wealth of studies that demonstrate its masculinizing effect on gonads (e.g., Piferrer et al, 1994;Wibbels and Crews, 1994). In the case of one previous study using Fadrozole with the turtle Trachemys scripta, a near pivotal incubation temperature was used in an attempt to increase the sensitivity of the embryos to Fadrozole , so it is possible that the higher incubation temperature (30°C) used in the current study obscured the effect.…”

“…This is exemplified by studies that block aromatase activity, which facilitates male development even under female-producing temperatures (reviewed in Warner, 2011). As a result, sexreversed males have similar gonad morphology to normal males, exhibit normal male behaviors, and are capable of spermatogenesis as adults (Elbrecht and Smith, 1992;Piferrer et al, 1994;Wennestrom and Crews, 1995;Chardard and Dournon, 1999;Shine et al, 2007;Warner and Shine, 2008;Warner et al, 2010).…”

Toxicity of the aromatase inhibitor letrozole to Japanese medaka (Oryzias latipes) eggs, larvae and breeding adults

“…There is increasing evidence in fish that multiple loci exist encoding at least two distinct isozymes of CYP19 that are differentially expressed in the brain and ovary (Kuhl et al, 2005). The potential for aromatase inhibitors to adversely affect sexual differentiation and reproduction in fish was demonstrated in an early study by Piferrer et al (1994), and similar results have been reported by Afonso et al (1999Afonso et al ( , 2000, Kitano et al (2000), Ankley et al (2002), andPanter et al (2002).…”

“…Sex inversions in fish exposed to EDCs have been reported previously (Yamamoto, 1953;Edmunds et al, 2000;Iwamatsu et al, 2000;Kitano et al, 2000) and may involve androgenic chemicals eliciting male characteristics in -Mercier et al, 1995;Belaid et al, 2001), red spotted grouper (Epinephelus akaara) (Li et al, 2005), chinook salmon (Oncorhynchus tshawytscha) (Piferrer et al, 1994), Japanese flounder (Paralichthys olivaceus) (Kitano et al, 2000) and zebrafish (Danio rerio) (McAllister and Kime, 2003;Andersen et al, 2004;Fenske and Segner, 2004) with different life stages exposed to AIs. However, some uncertainties remain.…”