Egfr Signaling Is a Major Regulator of Ecdysone Biosynthesis in the Drosophila Prothoracic Gland.

Understanding the mechanisms that determine final body size of animals is a central question in biology. In animals with determinate growth, such as mammals or insects, the size at which the immature organism transforms into the adult defines the final body size, as adult individuals do not grow [1]. In Drosophila, the growth period ends when the immature larva undergoes the metamorphic transition to develop the mature adult [2]. This metamorphic transition is triggered by a sharp increase of the steroid ecdysone, synthetized in the prothoracic gland (PG), that occurs at the end of the third instar larvae (L3) [3-6]. It is widely accepted that ecdysone biosynthesis in Drosophila is mainly induced by the activation of tyrosine kinase (RTK) Torso by the prothoracicotropic hormone (Ptth) produced into two pairs of neurosecretory cells that project their axons onto the PG [7, 8]. However, the fact that neither Ptth nor torso-null mutant animals arrest larval development but only present a delay in the larva-pupa transition [9-11] mandates for a reconsideration of the conventional model. Here, we show that Egfr signaling, rather than Ptth/torso, is the major contributor of ecdysone biosynthesis in Drosophila. We found that Egfr signaling is activated in the PG in an autocrine mode by the EGF ligands spitz and vein, which in turn are regulated by the levels of ecdysone. This regulatory positive feedback loop ensures the production of ecdysone to trigger metamorphosis by a progressive Egfr-dependent activation of MAPK/ERK pathway, thus determining the animal final body size.