TRPM8 thermosensation in poikilotherms mediates both skin colour and locomotor performance responses to cold temperature

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

Amphibian declines are sometimes correlated with increasing levels of ultraviolet radiation (UVR). While disease is often implicated in declines, environmental factors such as temperature and UVR play an important role in disease epidemiology. The mutagenic effects of UVR exposure on amphibians are worse at low temperatures. Amphibians from cold environments may be more susceptible to increasing UVR. However, larvae of some species demonstrate cold acclimation, reducing UV-induced DNA damage at low temperatures. Understanding of the mechanisms underpinning this response is lacking. We reared Limnodynastes peronii 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 ) UVBR. We measured the color of larvae and mRNA levels of a DNA repair enzyme. We reared larvae at 25°C in black or white containers to elicit a skin color 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. Skin darkening in larvae 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.

Novel observations of "freeze resistance" and dynamic blue and green dorsal coloration in frozen and thawing Dryophytes chrysoscelis

Freeze tolerant animals survive the winter by tolerating the freezing and thawing of up to 70% of body water and the respective cessation and resumption of essential functions including circulation and respiration during each freeze-thaw cycle. Cope's gray treefrog Dryophytes chrysoscelis is a freeze tolerant anuran that uses a system of cryoprotectants to prevent intracellular freezing and mitigate osmotic stress during freezing and thawing episodes. Morphological features were documented in D. chrysoscelis using a repeated freeze-thaw protocol. Dorsal skin in frozen frogs was distinctly blue and green before reverting to brown during thawing. The dorsal color change in frozen frogs does not function similarly to other known color change events in amphibians. The return to brown skin color in thawing animals coincides with recovery of vital functions in freeze tolerant frogs, suggesting that dorsal color change is an indicator of postfreeze recovery in D. chrysoscelis. We also provide evidence of "freeze resistance" in D. chrysoscelis. Two individuals did not freeze following three successive bouts of ice inoculation at -2.5°C and maintained brown dorsal color despite ice crystallization on the dorsum and contact with frozen substrate. Both frogs had similar plasma osmolality, circulating cryoprotectants, and incidence of cryoinjury compared to frogs that were frozen and thawed once or three times. Freeze resistance may be explained by physical changes in the skin including lipid accumulation and dehydration. This integrative study presents novel attributes of organismal freeze tolerance in D. chrysoscelis.