Ecdysteroid Control of Cell Cycle and Cellular Commitment in Insect Wing Imaginal Discs

The wing imaginal disc shows two different developmental responses to the steroid hormone 20-hydroxyecdysone (20E) in conjunction with cell cycle, and the responses depend on the developmental stage and the concentration to which it is exposed. The time of head capsule slippage (HCS) in late fourth instar, which signals the onset of molting process into the fifth instar, is the boundary for the appearance of 20E-dependency. Before HCS, 20E was not effective in maintaining the cell cycle, while, after HCS, 20E drove the cell cycle (20E-dependent cell cycle). In the 20E-dependent cell cycle, S phase was induced by 20E above a threshold concentration, whereas M phase was induced in a range of concentrations, above which it was suppressed. The 20E-dependent cell cycle first appeared concomitantly with the entrance of the discs into the reversible stage of pupal commitment. The composite data suggest that the control mechanism of cell cycle is altered in the pupally committed cells so that a high ecdysteroid titer to induce a stationary molt in young instars does not affect the cell division of uncommitted imaginal disc cells while that to induce pupal ecdysis suppresses it in the committed discs, which occurs before the onset of pupal differentiation.