Cold-induced skin darkening does not protect amphibian larvae from UV-induced DNA damage

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Many recent amphibian population declines are correlated spatially and temporally with increasing levels of ultraviolet radiation (UVR) in the atmosphere, particularly at high altitudes where temperatures are lower and UVR levels are higher. While the proximate causes of many high-altitude declines are novel disease, changing environmental factors that increase the susceptibility of hosts or the pathogenicity of microbes play an important role in disease epidemiology. UVR is a potent mutagen and immunosuppressant and the detrimental effects of UVR exposure on amphibians are compounded at low temperatures. This suggests that amphibians from cold environments may be more susceptible to increasing UVR. However, larvae of some amphibian species have demonstrated a capacity for cold-acclimation, which reduces the depressive effects of low temperature on UV-induced DNA damage accumulation. An understanding of the specific mechanisms underpinning this response are lacking. We conducted a series of experiments with Limnodynastes peronii larvae to elucidate the mechanism/s contributing to improved UVR tolerance with cold acclimation. We reared larvae in cool (15°C) or warm (25°C) waters before acutely exposing them to 1.5 h of high intensity (80 µW cm-2) UV-B radiation (UVBR). We measured the colour of each larva immediately before and after UVBR exposure and mRNA levels of the DNA repair enzyme CPD-photolyase at 12 h post-exposure. To examine the role of skin coloration in providing photoprotection to UVBR-exposed larvae, we reared larvae at 25°C in black or white containers to elicit a temperature-independent skin colour response, and then measured DNA damage levels in the skin and remaining carcass following UVBR exposure. Cold acclimated larvae were darker and displayed lower levels of DNA damage than warm-acclimated larvae. There was no difference in CPD-photolyase mRNA levels between cold-and warm-acclimated larvae following UV exposure. Larvae acclimated to dark backgrounds were darker than larvae acclimated to white backgrounds, but this did not reduce their accumulation of DNA damage following UVR exposure. Our results showed that skin darkening does not explain cold-induced reductions in UV-associated DNA damage in L. peronii larvae. Beneficial cold-acclimation is more likely underpinned by increased CPD-photolyase abundance and/or increased photolyase activity at low temperatures.