Response of larval Xenopus laevis to atrazine: Assessment of growth, metamorphosis, and gonadal and laryngeal morphology

Abstract

Larval Xenopus laevis were exposed to one of four concentrations of atrazine (0, 1, 10, or 25 microg/L, 11 replicate tanks per treatment, 60-65 larvae per replicate) dissolved in an artificial pond water (frog embryo teratogenesis assay- Xenopus [FETAX]) medium beginning 48 h after hatching until the completion of metamorphosis. Separate groups of larvae (six replicate tanks per treatment, 60-65 larvae per replicate) were exposed to estradiol (100 microg/L), dihydrotestosterone (100 microg/L), or ethanol vehicle control dissolved in FETAX medium. None of the treatments affected posthatch mortality, larval growth, or metamorphosis. There were no treatment effects on sex ratios except for estradiol, which produced a greater percentage of female offspring. Exposure to either estradiol or 25 microg atrazine/L increased the incidence of intersex animals based on assessment of gonadal morphology. Atrazine did not reduce the size of the laryngeal dilator muscle, a sexually dimorphic muscle in this species. We conclude that environmentally relevant concentrations of atrazine do not influence metamorphosis or sex ratios and do not inhibit sexually dimorphic larynx growth in X. laevis. The incidence of atrazine-induced intersex animals was small (<5%) and occurred only at the greatest concentration of atrazine tested, a concentration that is rarely observed in surface waters in the United States.

Intersexuality and the Cricket Frog Decline: Historic and Geographic Trends

“…Based on findings of intersexuality at very low atrazine concentrations, Hayes et al (2002, 2003) concluded that the widespread use of atrazine may have been a significant factor in amphibian declines. However, a recent study (Carr et al 2003) indicated a higher threshold for atrazine-induced intersexuality in frogs. Additional research is needed to resolve this issue.…”

“…At much lower concentrations, atrazine in combination with nitrate reduced growth of X. laevis (Sullivan and Spence 2003). Hayes et al (2002, 2003) associated hermaphrodism and gonadal dysgenesis in amphibians with very low aquatic concentrations of atrazine (0.1 μg/L), whereas Carr et al (2003) observed significantly increased intersexuality only at higher concentrations (25 μg/L).…”

Of Frogs & Rhetoric: The Atrazine Wars

“…Here are statements of two main findings from an industry-funded study (Carr et al, 2003). Notice that there are effects in one endpoint but no effects in the other:…”

“…On the other hand, studies finding no effects-all Syngenta funded-are represented as offering the final verdict on the matter. In the first Syngenta-funded study reporting no effects (Carr et al, 2003), references to the published studies reporting effects suggest conflict among the reports, which was not the case: "There have been conflicting reports regarding the potential for atrazine to influence reproductive development in amphibians at environmentally relevant concentrations" (Carr et al, 2003, p. 396). …”

“…Two scholars, James Carr, an environmental toxicologist at Texas Tech University and Werner Kloas, an environmental toxicologist with the Department of Ecophysiology and Aquaculture in Germany, coauthored papers funded by Syngenta that constructed atrazine has unlikely to harm and dismissed studies offering evidence to the contrary (Carr et al, 2003;Kloas et al, 2009). Yet both scientists are coauthors on papers that were not funded by industry.…”