Chronic exposures to monomethyl phthalate in Western clawed frogs

- Monomethyl phthalate (MMP) accelerates metamorphosis at high concentrations.
- High levels of MMP favors the male frog phenotype.
- MMP did not disrupt the expression of thyroid, sex steroid and cellular stress-related genes.

Polymer flexibility and elasticity is enhanced by plasticizers. However, plasticizers are often not covalently bound to plastics and thus can leach from products into the environment. Much research effort has focused on their effects in mammalian species, but data on aquatic species are scarce. In this study, Western clawed frog (Silurana tropicalis) embryos were exposed to 1.3, 12.3, and 128.7 mg/L monomethyl phthalate (MMP) until the juvenile stage (11 weeks) and to 1.3 mg/L MMP until the adult stage (51 weeks). MMP decreased survival, hastened metamorphosis, and biased the sex ratio toward males (2M:1F) at the juvenile stage without altering the expression of a subset of thyroid hormone-, sex steroid-, cellular stress- or transcription regulation-related genes in the juvenile frog livers. At the adult stage, exposure to MMP did not have significant adverse health effects, except that females had larger interocular distance and the expression of the heat shock protein 70 was decreased by 60% in the adult liver. In conclusion, this study shows that MMP is unlikely to threaten amphibian populations as only concentrations four orders of magnitude higher than the reported environmental concentrations altered the animal physiology. This is the first complete investigation of the effects of phthalates in a frog species, encompassing the entire life cycle of the organisms.