THERMAL SENSITIVITY OF DROSOPHILA-MELANOGASTER - EVOLUTIONARY RESPONSES OF ADULTS AND EGGS TO LABORATORY NATURAL-SELECTION AT DIFFERENT TEMPERATURES

We compared aspects of the thermal sensitivity of replicated lines of Drosophila melanogaster that had been evolving by laboratory natural s election at three selection temperatures: 16.5 degrees C (10+ yr), 25 degrees C (9+ yr), or 29 degrees C (4+ yr). The 16.5 degrees C and 25 degrees C lines are known to have diverged in fitness at 16.5 degrees C versus 25 degrees C and also in heat tolerance. We designed new expe riments to explore further possible shifts in thermal sensitivity of t hese lines. The optimal temperature for walking speed of adults was po sitively related to selection temperature, but differences among lines in thermal sensitivity of walking speed were small. Performance bread th was inversely related to selection temperature. Tolerance of adults to an acute heat shock was also positively related to selection tempe rature, but tolerance to a cold shock was not. Thus, fitness at modera tely high temperatures is genetically coupled with tolerance of extrem e high (but not of low) temperature. Knock-down temperature and walkin g speed at high temperature, however, were independent of selection te mperature. In contrast to adults, eggs from different lines had simila r heat and cold tolerance. Thus, long-term natural selection has led t o divergence in thermal sensitivity of some (but not of all) traits an d may have had more of an impact on adults than on eggs. Attempts to p redict evolutionary states in nature are, however, complicated because of the observed genetic correlations and the simple selection scheme.