Cell proliferation within the optic lobe anlagen is dependent on ecdysteroids during metamorphosis of the moth Manduca sexta. We use cultured tissues to show that ecdysteroids must be maintained above a sharp threshold concentration to sustain proliferation. Proliferation can be turned on and off repeatedly simply by shifting the ecdysteroid concentration to above or below this threshold. In subthreshold hormone, cells arrest in the G2 phase of the cell cycle. Ecdysteroid control of proliferation is distinguished from differentiative and maturational responses to ecdysteroids by requiring tonic exposure to the hormone and lower levels of 20-hydroxyecdysone, and by being sensitive to either 20-hydroxyecdysone or its precursor, ecdysone. These characteristics allow optic lobe development to be divided into two ecdysteroid-dependent phases. Initially, moderate levels of ecdysteroid stimulate proliferation. Later, high levels of 20-hydroxyecdysone trigger a wave of apoptosis within the anlage that marks completion of its proliferative phase.

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