Rethinking the ecdysteroid source during Drosophila pupal-adult development

Ecdysteroids, typified by 20-hydroxyecdysone (20E), are essential hormones for the development, reproduction and physiology of insects and other arthropods. For over half a century, the vinegar fly Drosophila melanogaster (Ephydroidea: Diptera) has been used as a model of ecdysteroid biology. Many aspects of the biosynthesis and regulation of ecdysteroids in this species are understood at the molecular level, particularly with respect to their secretion from the prothoracic gland (PG) cells of the ring gland, widely considered the dominant biosynthetic tissue during development. Discrete pulses of 20E orchestrate transitions during the D. melanogaster life cycle, the sources of which are generally well understood, apart from the large 20E pulse at the onset of pharate adult development, which has received little recent attention. As the source of this pharate adult pulse (PAP) is a curious blind spot in Drosophila endocrinology, we evaluate published biochemical and genetic data as they pertain to three hypotheses for the source of PAP 20E: the PG; an alternative biosynthetic tissue; or the recycling of stored 20E. Based on multiple lines of evidence, we contend the PAP cannot be derived from biosynthesis, with other data consistent with D. melanogaster able to recycle ecdysteroids before and during metamorphosis. Pub-lished data also suggest the PAP is conserved across Diptera, with evidence for pupal-adult ecdysteroid recycling occurring in other cyclorrhaphan flies. Further experimental work is required to test the ecdysteroid recycling hypothesis, which would establish fundamental knowledge of the function, regulation, and evolution of meta-morphic hormones in dipterans and other insects.

Automated summary

Ecdysteroids, such as 20-hydroxyecdysone, play crucial roles in the development, reproduction, and physiology of insects. The vinegar fly Drosophila melanogaster has long been studied as a model organism for ecdysteroid biology, and many aspects of ecdysteroid biosynthesis and regulation are well understood in this species. However, the source of a significant ecdysteroid pulse during the transition to adulthood remains unclear. Based on existing data, it is suggested that this pulse may be derived from the recycling of stored ecdysteroids rather than from biosynthesis. Further research is needed to test this hypothesis and improve our understanding of metamorphic hormone function in dipterans and other insects.