Date Published: July 3, 2017
Publisher: Public Library of Science
Author(s): Anna Mattsson, Björn Brunström, Bruce S Cushing.
Excess estrogen exposure of avian embryos perturbs reproductive organ development in both sexes and demasculinizes the reproductive behaviors of adult males. We have previously shown that these characteristic effects on the reproductive organs also can be induced by exposure of Japanese quail (Coturnix japonica) embryos to selective agonists of estrogen receptor alpha (ERα). In contrast, the male copulatory behavior is only weakly affected by developmental exposure to an ERα agonist. To further elucidate the respective roles of ERα and ERβ in estrogen-induced disruption of sexual differentiation, we exposed Japanese quail embryos in ovo to the selective ERα agonist 16α-lactone-estradiol (16αLE2), the selective ERβ agonist WAY-200070, or both substances in combination. The ERα agonist feminized the testes in male embryos and reduced cloacal gland size in adult males. Furthermore, anomalous retention and malformations of the Müllerian ducts/oviducts were seen in embryos and juveniles of both sexes. The ERβ agonist did not induce any of these effects and did not influence the action of the ERα agonist. Male copulatory behavior was not affected by embryonic exposure to either the ERα- or the ERβ-selective agonist but was slightly suppressed by treatment with the two compounds combined. Our results suggest that the reproductive organs become sexually differentiated consequent to activation of ERα by endogenous estrogens; excessive activation of ERα, but not ERβ, during embryonic development may disrupt this process. Our results also suggest that the demasculinizing effect of estrogens on male copulatory behavior is only partly mediated by ERα and ERβ, and may rather involve other estrogen-responsive pathways.
The sexual differentiation in birds is largely dependent on the plasma levels of gonadal estrogens; estrogens produced by the female embryo induce a female phenotype whereas the male phenotype develops at low estrogen concentrations. In Japanese quail (Coturnix japonica), the female embryo produces substantially higher levels of estradiol and estrone than the male embryo from at least embryonic day 5 (E5), which is at the onset of morphological sex differentiation of the gonads . Treatment of female chicken and Japanese quail embryos with inhibitors of aromatase or with estrogen receptor antagonists results in partial phenotypic sex-reversal manifested as formation of testis-like gonads, development of male secondary sex characteristics, lack of oviductal development and male-like growth of the cloacal gland in response to testosterone [2–5]. Conversely, treatment of male embryos with estrogens or experimental overexpression of aromatase cause feminization of the gonads such as development of an ovary-like left testis (ovotestis) and inhibited development of the right testis (only the left ovary and oviduct develop in females) [2, 6–8]. The gonadal feminization is transient and the testicles resume a fairly normal character later in life although structural and functional effects may still be found in the adult testis . In addition, the Müllerian ducts may not fully regress in estrogen-exposed male embryos and instead they develop into persisting misshapen oviduct-like structures [8, 10]. Furthermore, the cloacal gland may show reduced growth in response to circulating testosterone at sexual maturity . In females, embryonic exposure to excess estrogens results in anomalous retention of the right Müllerian duct, and both Müllerian ducts may develop into oviducts that are malformed and functionally impaired in the adult hen [12, 13]. The reproductive organ abnormalities seen after embryonic exposure to excess estrogen can also be induced by in ovo exposure to xenoestrogens such as 1-(2-chlorophenyl)-1-(4-chlorophenyl)-2,2,2-trichloroethane (o,p’-DDT) [14, 15], ethinylestradiol (EE2) [16, 17], bisphenol A (BPA)  and diethylstilbestrol [16, 18].
In the present study we explored the roles of ERα and ERβ in estrogen-induced disruption of avian sex differentiation by exposing Japanese quail embryos to the ERα-selective agonist 16αLE2 and the ERβ-selective agonist WAY. Reproductive organ development was affected by the ERα agonist, but not by the ERβ agonist. The male copulatory behavior was not affected by either of the ER agonists when administered alone, but was slightly suppressed by a combination of the two compounds.
The results from our present and previous studies suggest that ERα, but not ERβ, is involved in female differentiation of the ovary and Müllerian duct/oviduct in birds and that exogenous compounds that activate ERα may disturb this process and cause abnormal development of the reproductive organs in both sexes. The modest effect on male copulatory behavior by the ERα and ERβ agonists, separately and in combination, suggests that the organizational effects of estrogens on this behavior during development is mainly mediated by other estrogen-responsive pathways than those activated by ERα and ERβ.