Rapid cold hardening improves tissue viability and facilitates recovery of activity following nonlethal cold exposure in Drosophila melanogaster.

Low temperature is a significant stressor for small-bodied ectotherms like insects, and they employ a range of physiological mechanisms to mitigate damage caused by cold exposure. Rapid cold hardening (RCH) is a short-term acclimation response in which brief chilling (minutes to hours) dramatically improves tolerance to otherwise lethal temperatures. The ability of RCH to prevent lethality from low temperature exposure is well-studied and has been documented in many arthropod taxa. RCH can also protect against injuries incurred at nonlethal temperatures, but studies on the ability of RCH to protect against sublethal cold injury are limited, despite nonlethal cold exposure being more common in nature. Here, we test the hypothesis that RCH protects against sublethal cold injury in the model species Drosophila melanogaster. Specifically, we exposed flies to a nonlethal cold shock with and without an RCH pretreatment, and we assessed tissue viability, metabolic rate, macronutrient content, locomotor behavior, and reproduction after cold exposure. Relative to flies directly exposed to cold shock, flies pretreated with RCH had higher cell viability 24 h after treatment in both midgut and ovarian tissue. RCH also had longer-term organismal benefits; productivity in RCH females was higher than in non-hardened females for the first two days following treatment, and flies treated with RCH had higher activity 4-7 days after cold treatment, particularly in males. However, RCH did not preserve reproductive output in males or improve metabolic rate, macronutrient content, or short-term recovery rate from chill coma relative to flies exposed to nonlethal cold shock. Nonetheless, our results indicate that RCH does protect against specific types of sublethal cold injury, which further supports the ecological benefits of this trait.