Chernobyl black frogs reveal evolution in action

The accident at reactor four of the Chernobyl Nuclear Power Plant in 1986 generated the largest release of radioactive material into the environment in human history. The impact of the acute exposure to high doses of radiation was severe for the environment and the human population. But more than three decades after the accident, Chernobyl has become one of the largest nature reserves in Europe. A diverse range of endangered species finds refuge there today, including bears, wolves, and lynxes.

Radiation can damage the genetic material of living organisms and generate undesirable mutations. However, one of the most interesting research topics in Chernobyl is trying to detect if some species are actually adapting to live with radiation. As with other pollutants, radiation could be a very strong selective factor, favoring organisms with mechanisms that increase their survival in areas contaminated with radioactive substances.

Melanin protection against radiation:

Close to the damaged nuclear reactor, researchers detected several Eastern tree frogs (Hyla orientalis) with an unusual black tint. The species normally has a bright green dorsal coloration, although occasional darker individuals can be found.

Melanin is responsible for the dark color of many organisms. What is less known is that this class of pigments can also reduce the negative effects of ultraviolet radiation. And its protective role can extend to ionizing radiation too, as it has been shown with fungi. Melanin absorbs and dissipates part of the radiation energy. In addition, it can scavenge and neutralize ionized molecules inside the cell, such as reactive oxygen species. These actions make it less likely that individuals exposed to radiation will go on to suffer cell damage and increase their survival chances.

Between 2017 and 2019 they examined in detail the coloration of Eastern tree frogs in different areas of northern Ukraine. During those three years we analyzed the dorsal skin coloration of more than 200 male frogs captured in 12 different breeding ponds. These localities were distributed along a wide gradient of radioactive contamination. They included some of the most radioactive areas on the planet, but also four sites outside the Chernobyl Exclusion Zone and with background radiation levels used as controls.

The work reveals that Chernobyl tree frogs have a much darker coloration than frogs captured in control areas outside the zone. As researchers found out in 2016, some are pitch-black. This coloration is not related to the levels of radiation that frogs experience today and that we can measure in all individuals. The dark coloration is typical of frogs from within or near the most contaminated areas at the time of the accident.

The results of thsi study suggest that Chernobyl frogs could have undergone a process of rapid evolution in response to radiation. In this scenario, those frogs with darker coloration at the time of the accident, which normally represent a minority in their populations, would have been favored by the protective action of melanin.

The dark frogs would have survived the radiation better and reproduced more successfully. More than ten generations of frogs have passed since the accident and a classic, although very fast, process of natural selection may explain why these dark frogs are now the dominant type for the species within the Chernobyl Exclusion Zone.

Colouring gradient of the Eastern St. Anthony’s frog (Hyla orientalis) in northern Uk

The study of the Chernobyl black frogs constitutes a first step to better understanding the protective role of melanin in environments affected by radioactive contamination.

Pablo Burraco et al, Ionizing radiation and melanism in Chornobyl tree frogs, Evolutionary Applications (2022). DOI: 10.1111/eva.13476

Authors:

1. Germán Orizaola

   Investigador Ramón y Cajal, Universidad de Oviedo

2. Pablo Burraco  postdoctoral Juan de la Cierva Incorporación, Estación Biológica de Doñana (EBD-CSIC)

This article is republished from The Conversation under a Creative Commons license. Read the original article.