DAVIDOWITZ, G.*; NIJHOUT, H.F.; Univ. of Arizona; Duke Univ.: The Physiological Basis of Reaction Norms: Where Developmental Rate and Duration of Development Connect

The effects of temperature and nutritional quality on growth rate and body size are well known. We know little, however, of the mechanisms by which an organism translates variation in diet and temperature into reaction norms of body size or development time. We provide an endocrine-based physiological model that demonstrates how this translation occurs in the holometabolous insect Manduca sexta (Sphingidae). Three factors regulate body size and development time: growth rate, the timing of the cessation of juvenile hormone secretion (measured by the critical weight) and the timing of ecdysteroid secretion leading to pupation (measured by the interval to cessation of growth- ICG after reaching the critical weight). Together, the timing of these hormonal events determine the duration of the growth period. Reaction norms in body size and development time are a function of how growth rate interacts with these two traits. Ectotherms are generally smaller and develop faster at higher temperatures. Body size is smaller at higher temperatures, because the higher growth rate decreases the ICG, thereby decreasing the duration of the growth period. Development time is shorter at higher temperatures because the higher growth rate decreases the time required to attain the critical weight as well as the duration of the ICG. The temperature-dependence of these developmental factors completely account for the observed relationship between body size and environmental temperature.