Thermal acclimation of flies from three populations of Drosophila melanogaster fails to support the seasonality hypothesis.

In seasonal environments, natural selection should favor genotypes that acclimate to slow and predictable changes in temperature. Selective pressure for acclimation should be especially strong for animals that complete many generations per year, because seasonal warming or cooling causes offspring to experience different temperatures than their parents did. Here, we studied variation in acclimation capacity among three populations of Drosophila melanogaster. We used a reverse acclimation design to see whether developmental acclimation persisted throughout adulthood. Flies developed from fertilization to adulthood at either 16° or 26 °C. Then, flies either remained at the same temperature or moved to the other temperature for 7 days. We measured fecundity at seven temperatures ranging from 14° to 36°C. Genotypes from North Carolina and Vermont laid more eggs at 16 °C after spending the larval and adult stages at 16 °C, instead of 26 °C. In both populations, the benefit of acclimation to 16 °C during development was erased by acclimation to 26 °C during adulthood. In contrast to our prediction, genotypes from Indiana laid fewer eggs at 16 °C or 26 °C after developing at this temperature. Overall, these data provide only weak support for the models of optimal acclimation in seasonal environments.