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High levels of ecdysone which occur at the onset of metamorphosis cause differentiation of wing disc cells and their arrest in G210,18

High levels of ecdysone which occur at the onset of metamorphosis cause differentiation of wing disc cells and their arrest in G210,18. termination may be distinct in different disc regions. using the developing wing anlage, the wing disc, which is initially specified as a group of 30 cells that proliferate to yield an organ comprising 50,000 cells at the end of larval development2,4. At that point, growth in the animal essentially ceases due to a spike in ecdysone hormone, which causes animals to stop feeding, become pupae, and undergo metamorphosis5C7. Wing disc cells perform two final rounds of reductive cell divisions and exit the cell cycle roughly 24?h after pupa formation8C10. As the wing disc grows during larval development, wing disc cells undergo two separable processes: they grow (increase in biomass) and PluriSln 1 they proliferate (advance through the cell cycle and divide). Cell cycle progression per se does not lead to mass increase, and hence disc growth, as can be seen by genetic manipulations of cell cycle components11,12. For the disc to increase in mass, the cells need to increase in mass i.e., grow. However, a diploid cell can only increase in size within a limited range. Hence the 1000-fold increase in the size of the wing disc during larval development requires combined growth and proliferation of PluriSln 1 wing disc cells. The final size of the wing disc is set when cells stop growing and proliferating. The molecular mechanisms controlling growth termination are only starting to be understood. Several mechanisms have been ruled out. Disc cells do not count cell divisions to determine when to stop proliferating. If large fractions of the disc are killed genetically or with x-rays, the remaining cells compensate by proliferating more than usual, yielding normally sized organs13C15. Furthermore, PluriSln 1 accelerating or decelerating cell cycle progression in the wing disc leads to a normally sized organ composed of either more, smaller cells or fewer, larger cells, respectively11,12. Likewise, a mechanistic model where cells measure time has been excluded by slowing down disc growth using Minute mutations, and seeing that discs compensate by extending developmental time, thereby achieving a normal size16,17. These experiments have revealed that size sensing occurs at the level of compartments in the wing (e.g., the posterior versus anterior compartments)17, and that this occurs prior to pupation17. The hormone ecdysone plays an important, yet dichotomous role, in regulating wing disc PluriSln 1 size. High levels of ecdysone which occur at the onset of metamorphosis cause differentiation of wing disc cells and their arrest in G210,18. Hence this peak of ecdysone leads to termination of growth and proliferation in the wing. Indeed, a premature ecdysone peak causes premature termination of growth, yielding small animals19, whereas a delay in the ecdysone peak yields larger animals20,21. In contrast, the lower levels of ecdysone present during larval stages promote wing disc cell proliferation both in vivo3,16,18,22,23 and in cultured explants24,25. The downstream molecular mechanisms by which ecdysone influences wing cell proliferation are only partly understood22. Also unclear is whether ecdysone simply plays a permissive function in allowing wing cells to proliferate, or whether ecdysone levels control final wing disc size. We study here how ecdysone affects wing disc growth. To do so, we abolish endogenous ecdysone synthesis and reconstitute ecdysone signaling in vivo at different levels by feeding the bioactive form 20-hydroxy-ecdysone Arnt (20E) in a dose-controlled manner. We find that ecdysone supports wing disc growth in a dose-dependent manner, with higher levels of ecdysone enabling wing discs to reach a larger size. Consistent with this, the ability of ecdysone to support wing cell proliferation depends on the size of the wing disc: A fixed level of ecdysone signaling is able to promote proliferation of wing disc cells if they reside in a disc of small size, but not in a disc of.