REACTION NORMS OF MORPHOLOGICAL TRAITS IN DROSOPHILA - ADAPTIVE SHAPECHANGES IN A STENOTHERM CIRCUMTROPICAL SPECIES

Reaction norms of wing length, thorax length, and ovariole number were studied according to growth temperature in the circumtropical Drosoph ila ananassae, and compared to similar data from the cosmopolitan D. m elanogaster. In the two species convex reaction norms were observed, b ut they were not parallel and sometimes exhibited intersections either at high (wing) or at low (thorax) temperature. On average, D. ananass ae may be considered as a species with a bigger thorax but shorter win gs than D, melanogaster. The shapes of reaction norms were analyzed an d compared after quadratic polynomial adjustments. Significant differe nces were observed, in several cases between polynomial parameters, an d in all cases between characteristic points that is, Maximum Value (M V) and Temperature of Maximum Value (TMV). The wing/thorax ratio may a lso be considered as a specific trait related to wing loading. Major d ifferences were observed between the two species for the mean value an d the shape of the response curves of this trait. The main observation of this work was however a shift of TMVs for wing and thorax length a nd ovariole number in D. ananassae toward higher temperatures. These v ariations in the reaction norms corresponded to a shift in the species thermal range, suggesting that temperature adaptation was accompanied by a modification of the shape of the response curves.