Phenotypic plasticity of body pigmentation in Drosophila melanogaster: genetic repeatability of quantitative...

In Drosophila melanogaster, body pigmentation is a quantitative trait that depends on developmental temperature. When investigated over the whole thermal range of the species, pigmentation exhibits nonlinear reaction norms that differ among segments. The isofemale line method was used to analyse the genetic variability in two natural populations that affected the shape of reaction norms. Each line was considered as an experimental repeat, and polynomial reaction norms fitted to calculate the characteristic values (eg the coordinates of a maximum). In total, 20 lines from two geographically distant populations (France and India) were investigated at seven developmental temperatures (12-31 degrees C) in two successive generations (G2 and G3). We analysed the genetic repeatability (ie the correlation between generations) of three kinds of parameters: intraclass correlation coefficients (isofemale heritability), family means at different temperatures and the characteristic values of the reaction norms. For intraclass correlation, a low genetic repeatability was found. For family mean values grown at various temperatures, an overall positive and highly significant repeatability was found (r=0.55+/-0.024). Finally, a positive significant G2-G3 correlation was also the rule for the characteristic values of the reaction norms. Significant differences could be found between values describing either the trait or its plasticity, but with no general trend. A slightly higher repeatability was observed in the Indian population. These results show that, with a family selection design, the shape of the reaction norms might be modified in various ways.