Cold tolerance in Drosophila: adaptive variations revealed by the analysisof starvation survival reaction norms

Three species of Drosophila were investigated for their capacity to survive without food (starvation tolerance) at seven different temperatures rangin g from 0 to 25 degrees C. In all cases biphasic response curves (reaction n orms) were observed, corresponding either to special deleterious effects of cold or to a progressive exhaustion of reserves proportional to metabolic rate. The temperature at which survival was longest was called the threshol d temperature. The position of the threshold exhibited adaptive changes, ei ther due to acclimation in the same species, or to genetic variations evide nced between species. In D. melanogaster, adults grown at lower temperature (12 degrees C) were more tolerant to cold than adults grown at higher temp eratures (21, 25 or 30 degrees C). This acclimation process shifted, in an adaptive way, the position of the threshold temperature from 6.2 to 7.5 deg rees C. A comparison of three different species grown at a single developme ntal temperature (21 degrees C) revealed similar but greater adaptive diffe rences in their threshold temperature: 4.8 degrees C in the temperate D. su bobscura, 7 degrees C in the cosmopolitan D. melanogaster and 14.6 degrees C in the tropical D. ananassae. (C) 2000 Elsevier Science Ltd. All rights r eserved.