Hermaphroditic, demasculinized frogs after exposure to the herbicide atrazine at low ecologically relevant doses

Abstract

Atrazine is the most commonly used herbicide in the U.S. and probably the world. It can be present at several parts per million in agricultural runoff and can reach 40 parts per billion (ppb) in precipitation. We examined the effects of atrazine on sexual development in African clawed frogs (Xenopus laevis). Larvae were exposed to atrazine (0.01-200 ppb) by immersion throughout larval development, and we examined gonadal histology and laryngeal size at metamorphosis. Atrazine (> or =0.1 ppb) induced hermaphroditism and demasculinized the larynges of exposed males (> or =1.0 ppb). In addition, we examined plasma testosterone levels in sexually mature males. Male X. laevis suffered a 10-fold decrease in testosterone levels when exposed to 25 ppb atrazine. We hypothesize that atrazine induces aromatase and promotes the conversion of testosterone to estrogen. This disruption in steroidogenesis likely explains the demasculinization of the male larynx and the production of hermaphrodites. The effective levels reported in the current study are realistic exposures that suggest that other amphibian species exposed to atrazine in the wild could be at risk of impaired sexual development. This widespread compound and other environmental endocrine disruptors may be a factor in global amphibian declines.

Figure 1

Gonads of a control postmetamorphic male (A and C) and female (B and D) X. laevis. A and B show the entire dissected kidney–adrenal–gonadal complex preserved in Bouins' fixative. C and D show 8 μm of transverse cross-sections through the animals' right gonad stained with Mallory's trichrome stain. [Bar = 0.1 mm (A and B) and 10 μm (C and D)]. FB, fatbody; K, kidney. Arrows (in A and B) show the anterior and posterior ends of the animals' right gonads. The yellow color in A and B is a result of fixation in Bouins' fixative. Without fixation, the gonad is transparent. The ovary is distinguished by its greater length, lobed structure, and melanin granules. Although some specimens' ovaries lack pigment (especially atrazine-treated animals), testes never have melanin in this species. Histologically, the ovary is distinguished by the ovarian vesicle (hole in the center) along its entire length and the internal ring of connective tissue (in blue). Note the melanin granules (black) in the connective tissue in D.

Figure 2

An atrazine-treated hermaphrodite. The specimen shown was treated with 1 ppb atrazine. A shows the entire dissected kidney–adrenal–gonadal complex. B–E show 8 μm of transverse cross-sections (stained with Mallory's trichrome stain) through the areas indicated by the lines in A. [Bar = 0.1 mm (A) and 25 μm (B–E)]. FB, fatbody; K, kidney; O, ovary(ies); T, testis(es). Note the absence of pigment in the ovaries, which was typical of hermaphrodites.

Figure 3

Results of measurements of the left laryngeal muscle (M. dilator laryngis) in control males and females compared with atrazine-treated animals. In Exp. 1 (A), atrazine (≥1 ppb) reduced laryngeal size in males but did not affect females. Doses of 0.01 and 0.1 ppb did not have a significant effect. In Exp. 2 (B), 0.1–0.8 ppb atrazine did not have a statistically significant effect on laryngeal size but again, exposure to ≥1 ppb atrazine significantly reduced laryngeal size in males (P < 0.05). Laryngeal size was greater in animals from Exp. 2 compared with Exp. 1, suggesting a population difference in the absolute size of the larynges, but the relative sizes (male to female and atrazine-treated compared with controls) were similar within each experiment. C and D show two interpretations of the data by using analysis of the proportion of above-average males for both experiments. Atrazine exposure (≥1 ppb) significantly decreased the proportion of males that were at or above the mean for control males (G test; P < 0.05) and suggested a threshold effect at 1.0 ppb in which 80% of the exposed males were below average (C). Kendall's rank coefficient analysis (P < 0.01), however, suggested a relationship between dose and the proportion of affected males with a decrease in the proportion of normal males with increased dose (D). Note that control males were normally distributed with exactly 50% of the individuals above the mean in both experiments.

Figure 4

Effect of 46-day exposure to atrazine on plasma testosterone levels in sexually mature male X. laevis. Sexually mature males were housed individually. Experimental animals were treated every 3 days with 25 ppb atrazine, and controls were treated identically except without atrazine exposure. Control females are shown for comparative purposes. Letters above bars show statistical groupings (ANOVA, P < 0.05).