During the immature stages of development, the chief source of ecdysteroids is the prothoracic gland, a diffuse organ located in the thorax. In higher insects such as flies (Diptera) and bees (Hymenoptera), the prothoracic gland has become part of a composite structure called the ring gland. The precursor ecdysone or 3-dehydroecdysone is synthesized and immediately released into the blood. Then 20-hydroxymonoxgenase is converted to 20HE in target tissues such as epidermal cells, salivary glands, fat body, nervous system, gut, and imaginal discs.
During each stage of development, feeding and growth are followed by a sudden elevation of ecdysteroids, which induces animals to stop feeding and to engage in a new round of gene expression appropriate for the next stage. The epidermal cells begin to secrete a new layer of cuticle and to take back most of the old cuticle, recycling the chitin and protein recovered into the new layer. If the next stage is to be larval, ecdysteroids circulate with high levels of juvenile hormones, and a new set of larval characters is expressed. However, when larval development is complete, metamorphosis is signaled by short ecdysteroid pulses in the absence of juvenile hormones. Completely new structures are created as the insect undergoes the process of changing from a caterpillar into a reproductive, winged adult.
The actions of ecdysteroids can be observed almost immediately after their release as "puffs" on the large polytene chromosomes in the salivary glands of some flies. Even before ecdysteroids were chemically identified, Clever and Karlson noticed puffs within hours of injecting the natural hormone and suggested that 20HE acted to regulate gene expression. This was an excellent insight, for it is now known that ecdy-steroids have direct actions to either activate or suppress the expression of many genes via nuclear receptors and that the puffs correspond to gene loci at which transcription takes place.
A model explaining relationships between temporally distinct puffs was proposed by Michael Ashburner in the early 1970s. He found that just after ecdysteroid release, a set of early puffs could be observed, followed by early-late and late puffs. With the identification of ecdysteroid receptors, his model now views early and early-late puffs as evidence of transcriptional activity induced by ecdysteroid receptors (EcRs) and other nuclear receptors. The gene products resulting from these events then give rise to late puffs and also repress further transcriptional activity at early puffs. In this way, a complex but coordinated series of gene expression events occurs, initiated by EcRs.
Having been present throughout larval development, the prothoracic glands degenerate during metamorphosis. Nevertheless, ecdysteroids persist in the adult stage, where they play important reproductive functions. The source of ecdysteroids in adults remained a mystery for many years, although anecdotal accounts implicated the mobile oenocytes as a possibility. In the 1970s, Henry Hagedorn provided a breakthrough, showing that mosquito ovaries produce large quantities of 20HE and that the hormone is required for vitellogenesis, or yolk deposition in developing oocytes. The precise source of ecdysteroids is the follicle cell layer surrounding the oocyte. Since Hagedorn's initial finding, ecdysteroids have been identified also in the testes, where they are involved in sperm maturation.
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