Abstract
Understanding the mechanisms that determine the final body size of animals is a central question in biology. In insects, the level of the steroid hormone ecdysone dictates the end of the growth period by triggering metamorphosis. This process is thought to be induced by the activation of Ptth/torso signaling in the prothoracic gland (PG). Here we show that Egfr signaling, rather than Ptth/torso, is the major contributor of ecdysone biosynthesis in Drosophila. 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 level of ecdysone. This regulatory positive feedback loop ensures the production of ecdysone to trigger metamorphosis by a progressive Egfr-dependent activation of the MAPK/ERK pathway, thus determining the animal final body size.
Highlights
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) [3,4,5,6]
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]
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
Summary
Levels of the steroid hormones dictate the final size of insects by triggering metamorphosis. Steroid synthesis is thought to be mainly induced by Ptth/ torso signaling in the prothoracic gland. Cruz et al provide evidence that Egfr signaling, rather than Ptth/torso, is the major contributor of steroid biosynthesis in Drosophila. 2020, Current Biology 30, 1547–1554 April 20, 2020 a 2020 The Author(s).
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