Developmental constraints on an adaptive plasticity: reaction norms of pigmentation in adult segments of Drosophila melanogaster

Variation of dark pigmentation according to developmental temperature was i nvestigated in two geographic populations (France and India) with the isofe male line technique (20 lines for each population). The response curves cal led the reaction norms, were established in females for seven different seg ments: the mesothorax and abdomen segments 2-7 (Abd 2-7). In all cases the response curves were non-linear and had to be described either by a quadrat ic convex polynomial for thorax and Abd 2-5, or by a cubic polynomial for A bd 6 and 7. Among abdomen segments, increasing antero-posterior gradients w ere observed for several traits, including average pigmentation, overall ph enotypic plasticity, the temperature of minimum pigmentation, and the curva ture parameter of quadratic norms. Genetic correlations between abdomen seg ments were high when adjacent segments were considered, but became nil when more distant segments were correlated, suggesting that different pigmentat ion genes are expressed in the anterior and the posterior part of the abdom en. Characteristic values of reaction norms provided information either on trait value (i.e., the extension of pigmentation) or on plasticity. Correla tions between plasticity and pigmentation were generally low and non-signif icant, suggesting their genetic independence. The overall darker pigmentati on which is observed at low temperatures is assumed to be an adaptive plast icity. However. the differences which are evidenced among segments reveal s trong interactions with developmental genes. These interactions are less li kely to be a consequence of natural selection and are better interpreted as developmental constraints. The reaction norms analysis reveals the complex ity of these interactions and should help, in the future, in the identifica tion of the responsible thermosensitive genes.