Resistance to heat and cold stress in Drosophila melanogaster: intra and inter population variation in relation to climate

Genetic variation for resistance to high and low temperature stress and wing size was examined within and among four Drosophila melanogaster populations from temperate (Denmark and Italy) and subtropical areas (Canary Islands and Mali). The temperature of induction of the heat shock response was examined by conditioning flies to different high temperatures in the range 34 to 40 degrees C prior to exposing them to heat shock (41.5 degrees C for 0.5 h). Stress resistance appeared to be related to climate: populations from warm regions were the most heat tolerant and those from cold regions were the most cold tolerant. This trend suggests that natural selection in the wild at non-extreme temperature can lead to a correlated response in tolerance to extreme temperature. Wing size varied significantly and generally was larger for flies from more northerly populations. Populations varied genetically in all traits measured. Among traits, a positive correlation was present between heat-shock resistance with conditioning and resistance to cold, and the correlation was suggestive between heat-shock resistance with and without a conditioning treatment, but no correlation was indicated between cold resistance and heat resistance of non-conditioned individuals. Wing size was not correlated with any stress type. The results suggest that different groups of genes are involved in the resistance at extreme temperature ranges.