Big or fast: two strategies in the developmental control of body size

Abstract

Adult body size is controlled by the mechanisms that stop growth when a species-characteristic size has been reached. The mechanisms by which size is sensed and by which this information is transduced to the growth regulating system are beginning to be understood in a few species of insects. Two rather different strategies for control have been discovered; one favors large body size and the other favors rapid development.

Fig. 1

The insulin signaling network that links nutrition to growth and size. Nutrition in the form of amino acids stimulates insulin synthesis. Activation of the insulin receptor and receptor substrate (INR/INS) activates PKC and PKB/Akt via a multi-step pathway. PKC stimulates uptake of amino acids, which activate TOR. Insulin also activates TOR via PKB/Akt. TOR is inhibited by FOXO, which is inhibited by PKB/Akt. TOR stimulates protein synthesis via EIF4E transcriptional initiator, and causes cellular growth in tissues. In the prothoracic glands TOR stimulates ecdysone synthesis. Ecdysone is inhibited by FOXO when bound to Ultraspiracle (Usp) [16]. Inhibition of TOR by rapamycin inhibits growth and ecdysone synthesis. Ecdysone is also inhibited by juvenile hormone, which begins to decay at the critical weight, due to oxygen restriction. Thus, the ultimate control in this network resides in two environmental factors, nutrition and oxygen restriction

Fig. 2

Alternative control mechanism for body size. Sigmoidal growth curves with two different mechanisms that stop growth. a In the presence of juvenile hormone (JH), as in Manduca, ecdysone secretion cannot occur until JH has disappeared. The decline of JH begins when larvae pass the critical weight (CW). b In the absence of JH, as in Drosophila, ecdysone secretion follows attainment of the minimum viable weight (MVW) after a delay called the molt timer, whose mechanism is not yet understood. This leads to an earlier cessation of growth and a smaller body size than occurs when JH overrides the MVW mechanism