Juvenile hormone

Juvenile hormones (JHs) are a group of acyclic sesquiterpenoids that regulate many aspects of insect physiology. JHs regulate development, reproduction, diapause, and polyphenisms.

In insects, JH (formerly called neotenin) refers to a group of hormones, which ensure growth of the larva, while preventing metamorphosis. Because of their rigid exoskeleton, insects grow in their development by successively shedding their exoskeleton (a process known as molting).

Juvenile hormones are secreted by a pair of endocrine glands behind the brain called the corpora allata. JHs are also important for the production of eggs in female insects.

JH was discovered in 1965 and the first molecular structure of a final six was solved in 1967.

Most insect species contain only juvenile hormone (JH) III. To date JH 0, JH I, and JH II have been identified only in the Lepidoptera (butterflies and moths). The form JHB₃ (JH III bisepoxide) appears to be the most important JH in the Diptera, or flies. Certain species of crustaceans have been shown to produce and secrete methyl farnesoate, which is juvenile hormone III lacking the epoxide group. Methyl farnesoate is believed to play a role similar to that of JH in crustaceans.

Being a sesquiterpenoid, JH chemical structure differs significantly from the structure of other animal hormones. Some JH analogs have been found in conifers.

The primary control of juvenile hormone is by 1) the stimulation of the corpora allata by allatotropins short peptides which bind to G-protein coupled receptors, which signal the glands to produce JH, and 2), the inhibition of JH production by allatostatins. These fall into three classes: allatostatin A, allatostatin B, and allatostatin C. For a review of these control mechanisms see Stay and Woodhead 1993. Secondary control of the JH titre found in the haemolymph of the developing insect is metabolic inactivation of JH by JH-specific esterase and juvenile hormone epoxide hydrolase. During ecdysis the form of the old cuticle laid down before the next moult is controlled by the JH level in the insect. JH maintains a juvenile state. The level gradually decreases during the development of the insect, allowing it to proceed to successive instars with each molt.

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