Ecdysone

Definition

A steroidal prohormone secreted by the prothoracic glands of insects and certain other arthropods, serving as the immediate precursor to 20-hydroxyecdysone, the principal molting hormone that triggers ecdysis. Ecdysone itself has limited hormonal activity; it is converted by peripheral tissues (especially the fat body and epidermis) into the biologically active 20-hydroxyecdysone, which then initiates the transcriptional cascade that drives cuticle synthesis, apolysis, and the behavioral sequence of molting.

Etymology

From Greek ecdysis (ἔκδυσις), 'to strip off' or 'shed', referring to the molting process this hormone regulates

Example

In Drosophila melanogaster, a pulse of ecdysone at the end of the third larval instar induces chromosome puffs in salivary gland polytene chromosomes, visible evidence of the massive gene transcription required for pupariation and metamorphosis; recent work has also shown ecdysone pulses regulate temporal transitions in neural stem cell identity during larval brain development.

Synonyms

• α-ecdysone
• ecdysterone precursor

Related Terms

• 20-hydroxyecdysone
• ecdysteroid
• Ecdysis
• prothoracic gland
• molting
• Juvenile hormone
• apolysis
• polytene chromosome
• Metamorphosis

Usage Notes

Ecdysone is properly the prohormone, not the active molting hormone—a distinction often blurred in older literature where 'ecdysone' was used loosely for any ecdysteroid. Modern usage reserves ecdysone for the specific C27 compound (2β,3β,14α,22R,25-pentahydroxy-5β-cholest-7-en-6-one) and uses 20-hydroxyecdysone (or β-ecdysone, crustecdysone) for the active form. Ecdysteroid is the inclusive term for all related steroids with molting-hormone activity, including phytoecdysteroids from plants. The ecdysone → 20-hydroxyecdysone conversion is a critical regulatory step; tissues vary in their P450 monooxygenase activity, creating spatial and temporal gradients of hormone activity.