Relative importance of plastic vs genetic factors in adaptive differentiation:: geographical variation for stress resistance in Drosophila melanogaster from eastern Australia

1. Heritable clinal patterns for stress resistance traits have been described in a number of invertebrate species but patterns are usually characterized on populations reared under constant conditions. Here we examined the impact of simulated seasonal variation in temperature/photoperiod as well as constant conditions on stress resistance in eight Drosophila melanogaster populations from eastern Australia across a latitude range of 27 degrees. 2. Desiccation resistance was relatively higher under summer compared with winter/constant conditions, but this trait and starvation resistance did not exhibit clinal variation. Winter conditions increased cold resistance as measured by chill coma recovery time, and decreased heat resistance as measured by time to knock down in a vial. 3. Clinal patterns were evident for the thermal resistance traits regardless of conditions, and involved increased heat resistance and decreased cold resistance in tropical populations. Latitudinal patterns were steeper for cold resistance than for heat resistance. 4. To compare the relative impact of plastic vs genetic changes along the cline, differences in trait means were expressed relative to differences between populations from cline ends. For cold and heat resistance, differences between environmental conditions were approximately 1.5x greater than the heritable differences. 1