AbstractAnimals develop in unpredictable, changing environments. To do this, they adjust their development according to environmental conditions to generate plastic variation in traits, while also buffering against environmental change to produce robust phenotypes. However, how organ development is coordinated to accommodate both plastic and robust developmental responses is poorly understood. Here, we propose that the steroid hormone ecdysone coordinates both plasticity in organ size and robustness of organ pattern in the developing wings of the fruit fly Drosophila melanogaster. Ablating the prothoracic glands (PGX), which synthesise and secrete ecdysone, resulted in wing discs that were reduced in size and delayed in the progression of Achaete and Senseless pattern. These effects were rescued by adding ecdysone to the food. Further, while wing growth was driven by both ecdysone and nutrition, ecdysone alone induced the progression of wing patterning. To further explore this difference in response, we quantified wing growth and patterning in PGX larvae that were either fed on standard diet or starved of yeast across a range of ecdysone concentrations. Disc growth rates increased in a graded, linear manner with ecdysone concentration in starved larvae. In contrast, Achaete and Senseless patterning rates showed threshold responses regardless of diet. This means that ecdysone confers robustness by turning on patterning once it exceeds threshold concentrations, while inducing graded responses for disc growth, tuning growth to environmental conditions. This potentially represents a generalizable mechanism through which hormones coordinate plastic growth with robust patterning in the face of environmental change.Significance StatementTo survive changes in their environment, organisms adjust processes like growth to match the environment while ensuring that development processes necessary for correct body function remain insensitive to perturbation. For instance, organ size changes with environmental conditions like food availability and temperature. However, the specification of cell types within an organ, known as patterning, remains constant. How do animals coordinate variable and invariant developmental programs within the same organ? In this study, we define how a single hormone, the steroid hormone ecdysone, controls both variable growth and constant patterning in the developing wing. This is important, as it reveals a key pathway that allows insects to cope with environmental change and also highlights potential limits to surviving these changes.