Cold-shock and chilling tolerance in Drosophila

Adult flies of Drosophila melanogaster were examined for their tolerance to direct (cold-shock) and indirect chilling (chilling) injury. An exponential increase in mortality with low temperatures occurs. Chilling tolerance (T ch ) at 4 or 0°C was 6.3 days or 37.91 h for 50% survival (LT 50 ), respectively, while the cold-shock tolerance (LT 50 of T cs ) at −7°C was only 1.5 h. The increase in cold-shock tolerance (T cs ) at −7°C by acclimation at 4, or 0°C was rapid: the LT 50 increased about 50% with only 2 h of treatment in comparison with the highest LT 50 obtained by acclimation. An equilibrium point (T eq ), the optimal acclimation time for maximal survival of chilling and/or cold-shock, was calculated from the survival data of experiments designed to assess T ch and T cs . The T eq was found to be the half time of the acclimation period which resulted in 90% mortality at the chilling temperature (LT 90 of T ch ). Selection for tolerance to cold-shock or chilling over several generations resulted in a significant increase in T cs or T ch , respectively. Higher contents of glycogen and total proteins were observed in the cold selected lines than in the control line. Exposure to chilling temperatures eventually depleted the energy reserves, but the highest concentrations of triacylglycerols and proteins were observed at the acclimation time with the highest T cs . This suggests that systemic regulation in these energetic reserves may be an important part of the cold-hardening mechanisms to cope with the fluctuations of low temperatures causing cold-shock or chilling injury.