|Image by Hayley Trzinski|
The Princess and the Frog is a very fun and imaginative children’s story… but not when pesticides are involved. Have you ever wondered how dangerous pesticides can be? Well, pesticides can harm more than just pests and weeds, and in the case of frogs, many pesticides and herbicides are causing problems. Atrazine, a chemical commonly used as an herbicide, can cause reproduction in male African clawed frogs to be impossible. In some cases, atrazine is even turning some male frogs into females!
Tyrone Hayes, a biology professor at the University of California, Berkeley, and his research team looked at the effects of atrazine on African clawed frogs. The hypothesis of the researchers was to determine if exposure to atrazine would feminize or stop reproductive ability of male frogs. Tyrone Hayes and his team raised some frogs in atrazine dissolved in a weak ethanol solution and some in only a weak ethanol solution for a control group. Even though the ethanol solutions did not contain enough ethanol to impact the frogs, it is important to put the control group frogs in ethanol as well so both the treatment and control groups are equal in that way. Fertility in the frogs was later determined by looking at the number of developed embryos produced from atrazine-treated males and females, and from normal males and females.
Atrazine is an endocrine disruptor, meaning that it contains chemicals that can change the hormone systems in animals. Some endocrine disruptors block hormone receptors, causing the hormones needed for reproduction to stop working. Atrazine works as an endocrine disruptor by increasing the production and activity of aromatase, a chemical that turns testosterone into estrogen. This decreases the amount of testosterone and increases the amount of estrogen in the male frogs.
Tyrone Hayes and his team saw that aromatase was found in normal females and in atrazine-treated males, but not in normal males. The aromatase production caused a decrease in testosterone and an increase in estrogen in the atrazine-treated males. Subsequently, the atrazine-treated males' calls became less masculine, their sperm died, and they started forming characteristics such as female sex organs.
In the long run, atrazine could affect whole frog populations by skewing the sex ratio, meaning that there will be many more of one sex of frog than the other, making it hard to keep a healthy frog population. The main way that atrazine could skew the sex ratio of frogs is by changing their behavior. This starts by male frogs not being able to mate or by their fertility decreasing. Tyrone Hayes found that the behavior of male frogs treated with atrazine was different than the behavior of male frogs not treated with atrazine. Non-treated males out-competed atrazine-treated males for females and only two atrazine-treated males obtained correct mating posture. Also, non-treated males had much higher testosterone levels when around females than atrazine-treated males. This behavior change in male frogs affected by atrazine could cause fewer of those males to act like males, and more of them to act like females or to not reproduce at all.
Atrazine can skew the sex ratios of frog populations in other ways, too. African clawed frogs have the opposite type of sex determining chromosomes as humans. While human males have one Y and one X sex chromosome and human females have two X sex chromosomes, normal male African clawed frogs have two Z chromosomes and female African clawed frogs have one Z chromosome and one W chromosome. The sex ratio becomes skewed, in part, because even though some of the newly transitioned female frogs can successfully breed, they still have male genetics. When these newly transitioned female frogs mate with natural male frogs, all of the offspring will be males. This is because two frogs that both have original sex cells that are both Z’s create offspring that must inherit two Z chromosomes, making all of the babies male. This is dangerous for populations of frogs, because only one sex of frogs being created could lead to extinction of these creatures.
Although you may like the idea of crops being pest and weed free, there are many negative side effects to the dangerous pesticide chemicals, including changing the reproduction of frogs and even fish, reptiles, birds, and mammals, sometimes including humans. Even though the effects of these pesticides are pretty interesting, I don’t know about you, but I would rather read a story where frogs turn into princes instead of princesses.
Hayes, T., Khoury, V., Narayan, A., Nazir, M., Park, A., Brown, T., Adame, L., Chan, E., Buchholz, D., Stueve, T., & Gallipeau, S. (2010). Atrazine induces complete feminization and chemical castration in male African clawed frogs (Xenopus laevis) Proceedings of the National Academy of Sciences, 107 (10), 4612-4617 DOI: 10.1073/pnas.0909519107
Mnif, W., Hassine, A., Bouaziz, A., Bartegi, A., Thomas, O., & Roig, B. (2011). Effect of Endocrine Disruptor Pesticides: A Review International Journal of Environmental Research and Public Health, 8 (12), 2265-2303 DOI: 10.3390/ijerph8062265