THE EFFECT OF TEMPERATURE ON BODY SIZE AND FECUNDITY IN FEMALEDROSOPHILA MELANOGASTER: EVIDENCE FOR ADAPTIVE PLASTICITY

Abstract

Abstract.-The reaction norm linking rearing temperature and size in Drosophila melanogaster results in progressively larger flies as the temperature is lowered from 30 0e to l8°e, but it has remained unclear whether this phenotypic plasticity is part of an adaptive response to temperature. We found that female D. melanogaster reared to adulthood at 18°e versus 25°e showed a 12% increase in dry weight. Measurements of the fecundity of these two types of fly showed that 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 days 5 through 12 after eclosion at 25°e and days 7 through 17 at 18°e) was highest when the rearing and test temperatures were the same. The early fecundity advantage due to rearing at the test temperature was 25% at 18°e and 16% at 25°e, a result consistent with the overall phenotypic response to temperature being adaptive. 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 correlation. Another parallel is that both the temperature-induced and genetic effects are independent of total fecundity. By contrast, within the temperature treatments, the phenotypic correlation between early fecundity and longevity was positive, illustrating the danger of assuming that phenotypic and genetic correlations are similar, or even of the same sign.Key words.-Adaptive plasticity, body size, Drosophila melanogaster, fecundity, longevity, phenotypic correlation, temperature.Received August 14, 1996. Accepted May 7, 1997.The adaptive nature of phenotypic plasticity has been the subject of considerable research in recent years, prompting vigorous debate over how the reaction norms of such phenotypic responses evolve (see Via et al. 1995). However, before such questions can be addressed, the first priority in any project is to establish that the phenotypic plasticity is indeed adaptive and is not due to the intrinsic effects of shifting developmental conditions. A minimal requirement of adaptive phenotypic plasticity is that the phenotype and the environment interact to enhance individual fitness, that is, the phenotype induced by a particular set of environmental conditions results in a fitness gain that is specific to those conditions (for recent discussions see Sultan 1995; Gotthard and Nylin 1995).This minimal criterion of adaptation has been been found lacking in two recent studies of phenotypic plasticity of adult body size of Drosophila melanogaster in response to changes in temperature Zamudio et al. 1995). The reaction norm linking the body size of this species and rearing temperature defines a monotonic increase in body size as the rearing temperature is lowered from 30°C to 18°C (although at even lower temperatures this trend reverses slightly; see...

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