Meeting Abstract

92.5  Wednesday, Jan. 7  On the physiological determination of body size in Manduca sex: What is the critical weight? HELM, Bryan R*; DAVIDOWITZ, Goggy; University of Arizona bhelm@email.arizona.edu

In holometabolous insects, adult body size is determined by exponential growth that occurs during the larval stage. As a result, 90% of growth occurs during the last larval instar. Because growth rates are exponential, slight variation in the timing of the cessation of growth results in large differences in body size, making this a primary determinant of body size. In the tobacco hornworm, Manduca sexta, the cascade of physiological events leading to the cessation of growth is well established. The first of these events, the critical weight, defines when the corpora allata, the glands that synthesize and secrete juvenile hormone, switch off. Once it reaches the critical weight, the larva is committed to the cessation of feeding and pupation. Despite our understanding of the physiological events that regulate body size, the ultimate causes for the cessation of growth and determination of body size remain a mystery. In particular, little is known about what signals attainment of the critical weight to the organism. We investigated the physiological basis underlying critical weight by testing the hypothesis that nutrient accumulation in the the fat body signals attainment of the critical weight. We are surgically implanting fat body into caterpillars that have not yet reached their the critical weight, expecting to see a decrease in the time to pupation. Understanding the mechanism by which an organism ceases growth can ultimately provide an understanding of the physiological regulation of life histories as well and their response to selection.