Moulting
Part 2: Production of ecdysone from prothoracic glands
Terminology:
Ecdysone is a prohormone, converted into 20-hydroxyecdysone by fat body and epidermal cells. There are a number of different forms so the encompassing term, ecdysteroids is used. (Wigglesworth called it moulting hormone.)
Ecdysteroid is produced before and during the pharate period of the moulting cycle. It circulates in the haemolymph, able to contact all tissues. Ecdysteroids act on epidermis to trigger the series of apolysis -> mitosis -> new cuticle secretion.
- preparatory phase includes mitosis
- apolysis
- epicuticle deposition
- endocuticle
- preecdysial pigmentation
Correlation of cellular events during a molying cycle with the rise and fall of the ecdysteroid titre. (From Nijhout, 1994.)
Not all these events are concentration-specific, some events are “switched” and appear to become independent of the ecdysteroid concentration, however other processes are sensitive to the dropping phase in titre. For example:
- Programmed cell death in some neurons and muscles
- pigmentation
- secretion of eclosion hormone
This has been demonstrated by experimental disruptions:
- injection of ecdysteroid delays the onset of some of these physiological processes such as cell death and EH release
- premature reduction of titre by ligation causes premature cell death
Ecdysone and metamorphosis
During larval stages there is a single peak of ecdysteroid release.
In last larval instar many insects show 2 peaks:
- First, low peak: (a) reprograms the response pattern of the epidermis to secrete pupal cuticle, (b) Changes behaviour to prepare for pupation. Examples: “wandering larvae” of Diptera. Many insects stop feeding, void gut contents, burrow or climb, spin cocoon, etc.
- 2nd peak: Initiates the moult to the prepupa as per previous moults
The pattern of ecdysteroid secretion and the physiological responses are due to the absence of juvenile hormone (JH)
Prothoracic glands degenerate in the pupa (holometabola) or early adult (hemimetabola)
Ecdysteroids are released by ovaries of adult females and play a role in egg development, although of course they are no longer involved in moulting.
Hormone titers during the last two larval instars of Manduca sexta. The titers for PTTH, ecdysteroids and juvenile hormone are shown on a common time scale. Gray bars indicate the approximate timing and duration of the JHsensitive periods for larval versus pupal determination (L/P), for green versus black larval pigmentation (G/B), and for pupal versus adult determination (P/A). The epidermis and imaginal disks have independent times of commitment to pupal and adult development. (From Nijhout, 1994.)
Go on to part 3: Juvenile Hormone