DAVIDOWITZ, G*; NIJHOUT, H.F.; D’AMICO, L.J.: How is an insect the size that it is? How do physiological mechanisms change to produce genetic, environmental, and plasticity effects on body size?

Body size is one of the more important life history characters of many organisms; it is also well established that it is plastic. However, little is known about the developmental mechanisms that either regulate body size, or that translate different environmental signals that result in plasticity of body size. Here we demonstrate that body size in the tobacco hornworm (Manduca sexta) is regulated by three factors: growth rate, PTTH (prothoracicotropic hormone) delay time, and critical weight (the minimal weight at which further growth is not necessary for a normal time course to pupation). Together, these three factors explain over 95% of adult body size. We measured these factors in larvae reared on three diet qualities and at three temperatures and found that they varied with environmental conditions. A quantitative genetics study revealed significant genetic variation for size, critical weight and PTTH delay time, but not for growth rate. We further found that although each of these factors is highly correlated with body size, only plasticity of growth rate is highly correlated with plasticity of body size. These results suggest that the mechanisms that regulate body size are not the same as those that regulate plasticity of body size.