Specific effects of cycling stressful temperatures upon phenotypic and genetic variability of size traits in Drosophila melanogaster

In previous studies, the relationship between developmental temperature and phenotypic or genetic variability has been mainly investigated using different constant temperatures. Natural conditions, however, are characterized by daily thermal cycles, sometimes resulting in a periodic daily stress. Using the isofemale line technique, we examined the effects of daily cycles on body size in two French populations of Drosophila melanogaster. We used either cold stress (daily cycle 8-25 degree C, average 16.5 degree C) or heat stress (cycle 18-33 degree C, average 25.5 degree C), and the results were compared to those obtained at two constant temperatures, 17 and 25 degree C. In all cases, stressful regimes produced specific effects, the mean trait values being smaller than those observed under constant conditions. Significant differences were also found for the variance parameters. For the within-line variance, which mostly expresses an environmental component, the two cycling regimes produced similar outcomes - that is, a significant increase in individual variability. For the between-line, genetic variance, however, contrasting results were obtained: an increase with cold stress but a decrease with heat stress. With respect to constant-temperature conditions, evolvability (genetic CV) was increased by daily cold stress, but decreased by daily heat stress. Within-line correlations, between wing and thorax length, were stable and not affected by stress, whereas the between-line, genetic correlation was maximum under cold stress and minimum under heat stress. These results demonstrate that a periodic stress may induce specific effects with respect to permanent stress, and that heat and cold are not equivalent. A possible adaptive interpretation, related to the fact that temperate populations are certainly submitted to cold stress but not to heat stress, is discussed.