Phenotypic plasticity of abdomen pigmentation in two geographic populations of Drosophila melanogaster: male-female comparison and sexual dimorphism.

In Drosophila melanogaster male, the last abdominal tergites (A5-A6) are completely dark due to a strong internal constraint while, in female, all abdominal tergites (A2-A7) are phenotypically variable and highly plastic. Male A2-A4 are quite similar to those of female, but their plasticity was never investigated. In this paper, we compared the phenotypic plasticity of A2-A4 in both sexes in order to know if the major dimorphism (SD) expressed in male A5-A6 also extended toward the more anterior segments. We also compared two geographic populations living under very different climates in order to know if adaptive differences, previously observed in females also existed in males. With an isofemale line design, pigmentation variation according to growth temperature was investigated in the two populations from France and India. Male and female data were compared and sexual dimorphism (SD) analyzed in various ways. Reaction norms were quite similar in both sexes for A2 and A3, but clearly different for A4. Considering the total pigmentation (A2 + A3 + A4) males were darker than females at low temperatures and either identical to them (France) or lighter (India) above 25 degrees C. SD (male-female difference) was genetically variable among lines and significantly different among segments. Reaction norms of SD exhibited an overall decrease with temperature and also a significant difference among populations, suggesting a local adaptation of SD to thermal conditions. The three plastic segments in male (A2-A4) seem to react adaptively to the thermal environment more efficiently than the same segments in female, in agreement with the thermal budget hypothesis. To our knowledge, it is the first time that a SD trait exhibits an adaptive difference between geographic populations.