Phenotypic plasticity of body pigmentation in Drosophila: Influence of a developmental thermoperiodic regime in two sibling species

Abstract

Variation of body pigmentation according to growth temperature was analysed in two European populations (Bordeaux and Cordoba) of the two sibling species Drosophila melanogaster (Meigen) and D. simulans (Sturtevant). Results obtained at constant temperatures were compared with those observed under alternating thermal regimes of various amplitudes and with a daily periodicity of 12-12h. Abdominal pigmentation in segments 5, 6 and 7, and of their sum, was investigated in both species in females only. Thoracic trident pigmentation was studied in males and females but in Drosophila melanogaster only. Thoracic pigmentation exhibited curvilinear convex reaction norms under constant conditions, with a darker pigmentation in the population (Bordeaux) living in a colder climate. No specific effects of alternating regimes were seen. For abdomen pigmentation, slightly different reaction norms were observed under constant conditions between Bordeaux and Cordoba strains. There were also slight differences between the two species. Alternating temperatures produced a significant decrease in average pigmentation, roughly proportional to the thermal amplitude. Alternating stressing conditions induced also an increase in the phenotypic variability of abdominal pigmentation, especially in segments 6 and 7. Finally, because an increase of growth temperature results in a monotonically decreasing reaction norm of both abdomen pigmentation in females and a size-related trait, the wing/thorax ratio, we found a linear relationship between the two variables, with a very high overall correlation at constant temperatures (r=0.99). The correlation was slightly less under alternating conditions, due to a broader dispersal of the experimental values around the regression lines.