ECDYSTEROID CONTROL OF CELL-PROLIFERATION DURING OPTIC LOBE NEUROGENESIS IN THE MOTH MANDUCA-SEXTA

Cell proliferation within the optic lobe anlagen is dependent on ecdys teroids during metamorphosis of the moth Manduca serta. We use culture d tissues to show that ecdysteroids must be maintained above a sharp t hreshold concentration to sustain proliferation. Proliferation can be turned on and off repeatedly simply by shifting the ecdysteroid concen tration to above or below this threshold. In subthreshold hormone, cel ls arrest in the G(2) phase of the cell cycle. Ecdysteroid control of proliferation is distinguished from differentiative and maturational r esponses to ecdysteroids by requiring tonic exposure to the hormone an d 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-depend ent phases. Initially, moderate levels of ecdysteroid stimulate prolif eration. Later, high levels of 20-hydroxyecdysone trigger a wave of ap optosis within the anlage that marks completion of its proliferative p hase.