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The size and shape of organs is species specific, and even in species in which organ size is strongly influenced by environmental cues, such as nutrition or temperature, it follows defined rules. Therefore, mechanisms must exist to ensure a tight control of organ size within a given species, while being flexible enough to allow for the evolution of different organ sizes in different species. We combined computational modeling and quantitative measurements to analyze growth control in the Drosophila eye disc. We find that the area growth rate declines inversely proportional to the increasing total eye disc area. We identify two growth laws that are consistent with the growth data and that would explain the extraordinary robustness and evolutionary plasticity of the growth process and thus of the final adult eye size. These two growth laws correspond to very different control mechanisms and we discuss how each of these laws constrains the set of candidate biological mechanisms for growth control in the Drosophila eye disc. [ABSTRACT FROM AUTHOR]

The article focuses on a Shh/Gli-driven three-node timer underlying the sequential generation of motor neurons (MNs) and serotonergic neurons in the brainstem. It mentions the timer is founded on temporal decline of Gli-activator and Gli-repressor activities established through down-regulation of Gli transcription. It also mentions that how time is reliably encoded during the sequential specification of neurons.