Chronic Effects of Cadmium on Gonad Differentiation of the Spot Frog (Pelophylax nigromaculata) Larvae

Cadmium (Cd), one of the most common endocrine disruptors (EDCs), plays an important role in sex differentiation by blocking receptors of sex steroid hormones in amphibians. To analyse the influence of Cd on gonadal sex differentiation of spot frog (Pelophylax nigromaculata) larvae, the fertilized eggs were exposed to Cd (100, 10, 1 and 0.1 μg/L) in water until complete metamorphosis. We observed sex ratio and gonadal condition, detected the expression and location of estrogen receptor (ER) and metallothioneins (MTs) on the gonad cells by immunohistochemical method. Some intersex gonads (the same gonad has testicular and ovarian-like elements) in Cd treatment at 10, 1 and 0.1 μg/L were observed. Compared to control, female/male ratios at Cd treatment levels (1 and 0.1 μg/L) were significantly different (p < 0.05). ER and MTs were positively expressed in the cytoplasm and nuclei of gonad, and the expression of ER and MTs did not show a monotonic linear relationship in the Cd treatment. In the relatively lower concentration (10, 1 and 0.1 μg/L), ER and MTs expressions were stronger, while it appeared to be weak immunoreactivity in higher concentration (100 μg/L). The expression and location of ER and MTs on the gonad cells and sex ratios were affected by Cd, indicating that Cd has estrogenic effect on P. nigromaculata.