Chernobyl Radiation Caused Frogs to Genetically Mutate, Turning Black

The radioactive fallout of the 1986 Chernobyl nuclear reactor disaster has resulted in the rapid evolution of species nearby. According to a study published in the journal Evolutionary Applications, the radiation from the Chernobyl accident has resulted in the changing of the skin coloration of the Eastern tree frog (Hyla orientalis) in Ukraine, turning from green to black. "Skin coloration was darker in localities closest to areas with high radiation levels at the time of the accident, whereas current radiation levels seemed not to influence skin coloration in Chernobyl tree frogs," wrote the authors in the paper. "Tree frogs living within the Chernobyl Exclusion Zone had a remarkably darker dorsal skin coloration than frogs from outside the Zone."

The number four reactor at the Chernobyl nuclear power plant melted down and exploded on 26 April 1986, with an open-air fire burning in the reactor core for several days. This resulted in airborne radioactive contaminants being released and blown across Ukraine and much of nearby Europe. These radioactive isotopes, mostly fission products, release ionizing radiation, which can collide with DNA and damage it, causing genetic mutations. These mutations can lead to cancers and death, malformed offspring, or, in rare cases, non-harmful mutations that get passed onto the next generation. This is a form of radiation-driven evolution. According to the paper, strong selective factors, such as Chernobyl's radiation or environmental pollutants, have been previously seen to induce fast adaptive responses, including in fish and plants. In the case of the Ukrainian tree frogs, the researchers found that the darker coloration may not have necessarily been due to a random mutation, but instead an adaptive response to protect the frogs from the abundant radiation. "Dark coloration is known to protect against different sources of radiation by neutralizing free radicals and reducing DNA damage, and, particularly melanin pigmentation has been proposed as a buffering mechanism against ionizing radiation. Our results suggest that exposure to high levels of ionizing radiation, likely at the time of the accident, may have been selected for darker coloration in Chernobyl tree frogs," wrote the authors in the paper.

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Melanin is responsible for the dark skin and eye coloration of many animals, including humans, but can also reduce the negative effects of radiation. Melanin absorbs and dissipates part of the radiation energy, protecting us from ultraviolet light, and has been shown to protect against other ionizing radiation in fungi. "In addition, it can scavenge and neutralize ionized molecules inside the cell, such as reactive oxygen species," wrote the authors in a Conversation article. "These actions make it less likely that individuals exposed to radiation will go on to suffer cell damage and increase their survival chances." In essence, as opposed to randomly mutating to turn black due to the radiation, the frogs may have instead evolved resistance to the radiation. "Further studies are needed to determine the underlying mechanisms and evolutionary consequences of the patterns found here," wrote the authors. "We acknowledge the limitations of our field approach regarding the lack of field-based experiments and the small sample size used in the experiment testing for lability in coloration."