Sexually dimorphic expression of sexual differentiation genes in the internal genital organs of Japanese quail embryos

Abstract

Japanese quail (Coturnix japonica) is an avian model used to evaluate the reproductive and developmental toxicity of chemicals. The National Institute for Environmental Studies (NIES) of Japan established a strain of Japanese quail, NIES-L, which may be a better model because of its highly inbred characteristics. To understand sexual differentiation of the reproductive organs and the value of using NIES-L quails for avian toxicity assessment, we profiled estradiol and androgen plasma levels by enzyme-linked immunosorbent assay; the mRNA levels of estrogen receptor-α (ERα), ERβ, and androgen receptor (AR) in the gonads, Müllerian ducts, Wolffian ducts; and the mRNA levels of steroidogenic enzymes, cholesterol side chain cleavage enzyme (P450_scc), 17α-hydroxylase/C_17-20 lyase (P450_{17α, lyase}), 3β-hydroxysteroid dehydrogenase (3β-HSD), 17β-hydroxysteroid dehydrogenase (17β-HSD), and aromatase (P450_arom), anti-Müllerian hormone (AMH), and AMH receptor type 2 (AMHR2) in the gonads of NIES-L Japanese quails on embryonic days 9, 12, and 15 using a real-time quantitative PCR method. The plasma estradiol concentration was higher in females than males on these embryonic days, but no sex difference was found in the plasma androgens. The mRNA levels of all examined steroidogenic enzymes were significantly higher in female than male embryos. In particular, the P450_arom mRNA levels showed a striking sex difference: P450_arom was expressed in female but not male gonads. In contrast, the AMH and AMHR2 mRNA levels in the gonads were higher in males than females. The ERα, ERβ, and AR mRNA levels increased in the left female gonad and peaked on embryonic day 15, but not in the left and right male gonads; therefore, there was a female-biased sex difference. The ERα, ERβ, and AR mRNA levels in the left Müllerian duct, but not in the right Müllerian duct, of females increased and peaked on embryonic day 15, which resulted in asymmetric mRNA levels. The Wolffian ducts expressed ERα, ERβ, and AR in both sexes, and no sex difference or asymmetry of mRNA levels was found. The information obtained from this study helps elucidate the molecular endocrinological basis of sexual dimorphism formation of reproductive organs and clarify the value of NIES-L quails for toxicity assessment.

Keywords: Androgens; Anti-Müllerian hormone; Estradiol; Japanese quails; Sex steroid receptors; Steroidogenic enzymes.