Time to Molt

Science Signaling  06 Sep 2011:
Vol. 4, Issue 189, pp. ec247
DOI: 10.1126/scisignal.4189ec247

How insects sense their body size and determine when to undergo a molt is a fundamental, yet unsolved, question in developmental biology. Reaching the “critical weight,” which is specific to each developmental stage, induces secretion of the steroid hormone ecdysone to trigger molting. Mechanisms by which body size is sensed and controlled are not completely understood, but oxygen availability is one factor that influences body size. Callier and Nijhout looked at the role of the tracheal system, which delivers oxygen directly to tissues, in controlling larval body size and molting times in the tobacco hornworm Manduca sexta. Measurement of tracheal system volume and imaging of gut tracheae indicated that the tracheal system increased in volume and size by a discrete amount at each molt, so that larvae began each developmental stage (instar) with a tracheal system of a set size that did not increase during that instar. In contrast, the body mass and size of larvae increased throughout the instar. Respiration rates of third, fourth, and fifth instar larvae initially increased linearly with body mass but remained constant after larvae reached the critical weight. Because oxygen diffuses passively from the trachea into nearby tissues, tissues become hypoxic as they increase in size. This suggested that the critical weight could correspond to the size at which oxygen became limited. Indeed, fourth and fifth instar larvae grown under hypoxic conditions from the beginning of the respective instar molted at weights lower than those of control larvae grown under normoxic conditions. Thus, under normal conditions, a nongrowing tracheal system that is stage-specifically scaled at the beginning of each instar may provide a means, based on oxygen availability, for limiting growth or for sensing body size, thereby determining when it is time to molt.

V. Callier, H. F. Nijhout, Control of body size by oxygen supply reveals size-dependent and size-independent mechanisms of molting and metamorphosis. Proc. Natl. Acad. Sci. U.S.A. 108, 14664–14669 (2011). [Abstract] [Full Text]