Ever since the blanket of ozone that protects the planet from hazardous UV radiation began to dwindle, there have been concerns about the risks of increased UV exposure. And humans are not the only animals that are likely to suffer. André Schuch from the Federal University of Santa Maria, Brazil, explains that animals that are at the larval life stage are particularly vulnerable to the damaging effects of the radiation. ‘The global increases in UVB radiation associated with stratospheric ozone depletion have been considered a leading hypothesis for amphibian decline’, says Schuch. Yet, little was known about the effects of UV radiation on DNA in developing tadpoles. ‘It has been demonstrated that exposure to UVB radiation can cause spinal curvature and reduced growth and development’, says Schuch, so he and his colleagues Sonia Cechin and Elgion Loreto set about finding out just how damaging UV radiation is for tadpole DNA.
Explaining that UV radiation directly attacks genes by linking adjacent thymine or cytosine bases in the DNA sequence – causing the gene to be miscopied the next time that the cell divides or altering the way that the gene is converted into a protein during development – Schuch and his colleagues decided to find out how well Montevideo treefrog (Hypsiboas pulchellus) tadpoles could repair UV damage to their genes. First, Schuch, Victor Lipinski, Mauricio Santos and Caroline Santos went fishing for tadpoles in a pond on the university campus. Returning to the lab with approximately 1000 recently hatched tadpoles, Schuch exposed some of the animals to UVA radiation and another group to UVB for different lengths of time. Next, knowing that light activates one specific DNA repair mechanism – where a protein called photolyase repairs the damage by detaching the link between the DNA bases – Schuch bathed the animals in light. Finally, Sinara Jardim extracted DNA from the tadpoles’ tails, ready for Schuch to measure the amount of damage that the animals had sustained.
Analysing the DNA, Schuch could see that the tadpoles were very sensitive to UVB: in addition to sustaining higher levels of damage, the tadpoles’ DNA repair rate was slower. ‘The [photolyase] enzymes left almost 40% of UVB-generated DNA lesions in the genome even after 3 h of photoreaction treatment’, says Schuch. And when he measured the effects of the radiation on the cell death rates, it was clear that the radiation was extremely toxic. ‘Taken together, the results show that these DNA repair pathways are less efficient than we expected, indicating that treefrog tadpoles could be severely affected when exposed to agents that induce large distorting DNA lesions’.
However, the news was not all bad for the tiny developing animals. Wondering whether the tadpoles could detect and evade the damaging radiation, Schuch shaded one half of the tadpoles’ Petri dish home with a black-plastic cover and filmed the tadpoles’ reactions as he bathed the other half of the Petri dish with light of different wavelengths. Noticing that the tadpoles were content to bask in light that lacked UV radiation, Schuch was impressed to see the animals surge for the shade when he first turned on the UVB and then UVA wavelengths. ‘This behaviour should complement the low DNA repair efficiency of the tadpoles’, says Schuch, who is keen to learn more about the impact of deforestation at the treefrog's breeding sites on the tadpoles’ UV exposure.
