THE EFFECT OF TEMPERATURE ON BODY-SIZE AND FECUNDITY IN FEMALE DROSOPHILA-MELANOGASTER - EVIDENCE FOR ADAPTIVE PLASTICITY

The reaction norm linking rearing temperature and size in Drosophila m elanogaster results in progressively larger flies as the temperature i s lowered from 30 degrees C to 18 degrees C, but it has remained uncle ar whether this phenotypic plasticity is part of an adaptive response to temperature. We found that female D, melanogaster reared to adultho od at 18 degrees C versus 25 degrees C showed a 12% increase in dry we ight. Measurements of the fecundity of these two types of fly showed t hat the size change had no effect on lifetime fecundity, regardless of the adult test temperature. Thus the phenotypic plasticity breaks the usual positive correlation between body size and fecundity. However, at a given temperature, early fecundity (defined as productivity for d ays 5 through 12 after eclosion at 25 degrees C and days 7 through 17 at 18 degrees C) was highest when the rearing and test temperatures we re the same. The early fecundity advantage due to rearing at the test temperature was 25% at 18 degrees C and 16% at 25 degrees C, a result consistent with the overall phenotypic response to temperature being a daptive. This conclusion is further supported by the finding that the temperature treatments resulted in a trade-off between early fecundity and longevity, a trade-off that parallels the known genetic correlati on. Another parallel is that both the temperature-induced and genetic effects are independent of total fecundity. By contrast, within the te mperature treatments, the phenotypic correlation between early fecundi ty and longevity was positive, illustrating the danger of assuming tha t phenotypic and genetic correlations are similar, or even of the same sign.