Phenotypic plasticity and developmental temperature in Drosophila: Analysis and significance of reaction norms of morphometrical traits

In ectotherm species like Drosophila, morphometrical quantitative traits exhibit a large amount of phenotypic plasticity as a consequence of developmental temperature. Generally the response curves called reaction norms, are non-linear and exhibit either single maxima or minima, or a sigmoid shape. Up to now, such complex shapes were mainly considered as non-adaptive, reflecting interactions between internal developmental constraints and the environment. We show that the general, all purpose method of polynomial adjustment provides a convenient way for the description of the norms. From the polynomial parameters, characteristic values with an obvious biological significance (e.g. the temperature of a maximum size), can be calculated and used for comparing populations or species. Experimental data on body pigmentation and body size show that the shapes of the reaction norms are genetically variable and suggest adaptive responses to the environment. The possible occurrence of specific regulatory genes acting on norm shape will be a matter for further investigations.