In Drosophila melanogaster females, pigmentation of the abdominal tergites exhibits a large phenotypic variability which depends on growth temperature. The phenotypic plasticity of this trait was investigated in a European population, using isofemale lines, by rearing the larvae and pupae at 6 different temperatures ranging from 14 to 31° C. For the last three abdominal segments (5, 6 and 7), the dark pigmented area decreases when growth temperature increases. However, the shapes of the response curves are significantly different, as shown by analysing slope variations, suggesting that genes interact differently in regulating the phenotypic expression of successive segments.
Also, the correlation between the values of the same line at two temperatures exhibited a decrease proportional to the temperature interval considered. Genetic variability between lines was analyzed by calculating the coefficient of intraclass correlation, t. This parameter was not influenced by growth temperature and, according to the segment, ranged from 0.40 to 0.54. Such high values suggest a large proportion of non-additive effects. A multivariate analysis helped to visualize the diversity of the reaction norms among lines. Overall pigmentation variations are presumed to have an adaptive significance, related to the thermal balance of adult flies; on the other hand, the differences between segments could reflect developmental constraints without a direct adaptive value.