Insulin-like growth factor (IGF)-like peptide and 20-hydroxyecdysone regulate the growth and development of the male genital disk through different mechanisms in the silkmoth,Bombyx mori

It is well established that ecdysteroids play pivotal roles in the regulation of insect molting and metamorphosis. However, the mechanisms by which ecdysteroids regulate the growth and development of adult organs after pupation are poorly understood. Recently, we have identified insulin-like growth factor (IGF)-like peptides (IGFLPs), which are secreted after pupation under the control of 20-hydroxyecdysone (20E). In the silkmoth, Bombyx mori, massive amounts of Bombyx-IGFLP (BIGFLP) are present in the hemolymph during pupal-adult development, suggesting its importance in the regulation of adult tissue growth. Thus, we hypothesized that the growth and development of adult tissues including imaginal disks are regulated by the combined effects of BIGFLP and 20E. In this study, we investigated the growth-promoting effects of BIGFLP and 20E using the male genital disks of B. mori cultured ex vivo, and further analyzed the cell signaling pathways mediating hormone actions. We demonstrate that 20E induces the elongation of genital disks, that both hormones stimulate protein synthesis in an additive manner, and that BIGFLP and 20E exert their effects through the insulin/IGF signaling pathway and mitogen-activated protein kinase pathway, respectively. These results show that the growth and development of the genital disk are coordinately regulated by both BIGFLP and 20E.