The youthful hormone is a Hormone which controls the post-embryonic development at the Insectes. It owes its name with the fact that it maintains the characters youthful, by supporting the driven larval and by delaying the metamorphosis. However, it does not intervene only in the larvae, since it has also a very important role in the adult insect, where it controls the reproduction, in particular the Vitellogenèse and the Ovogenèse in the female.

It is almost impossible to evoke the role of the youthful hormone, during the development as at the time of the reproduction, without mentioning another hormone, the Ecdysone, with which it has many interactions. But the exact mode of action of the youthful hormone, contrary to that of the ecdysone, was not bored yet up to date.

Many mimetic substances of the youthful hormone were discovered, in extracts of plants on the one hand, but also thanks to the chemical synthesis on the other hand. Some of these modulating endocriniens, called juvénoïdes , were used successfully like Insecticide S very specific, having an almost null toxicity for the vegetation and the other animals.

Nature of the youthful hormone

One can find in the insects several molecular forms of youthful hormone. Generally, they are derivatives sesquiterpénoïdes close to the methyl Ester of the farnésoïque acid (methyl-farnésoate or MF) and having one or two groupings epoxy S. the first described forms were named in the order of their discovery: JH1, JH2 and JH3 (JH coming obviously from the English abbreviation for youthful hormone ). Then other forms were found, of which the denomination any more did not follow same logic, in particular the JH0 (to pronounce JH-zéro) found in eggs of Lépidoptère S or the JHB3 (JH3-bisépoxyde) found in the flies. The majority of the insects contain only JH3, except the flies which also have JHB3 and the lépidoptères which can have several JH (JH0, 1,2 or 3) within the same species.

Form of the various JH:

• JH1 (or JH I): youthful hormone with 18 carbons and 1 epoxy: C₁₈H₃₀O₃
• JH2 (or JH II): youthful hormone with 17 carbons and 1 epoxy: C₁₇H₂₈O₃
• JH3 (or JH III): youthful hormone with 16 carbons and 1 epoxy: C₁₆H₂₆O₃
• JH0 : youthful hormone with 19 carbons and 1 epoxy: C₁₉H₃₂O₃
• JHB3 : youthful hormone with 16 carbons and 2 epoxies: C₁₆H₂₆O₄

Metabolism of the youthful hormone

Biosynthesis is carried out in the bodies allates, which are endocrine glands close to the brain, often associated with the cardiac bodies, with which they constitute a body presenting many analogies with the Hypophyse of the vertebrate ones.

The way of biosynthesis is relatively well-known because it uses homologous enzymes with those which intervene in the biosynthesis of the Stérol S in the other animals. But the insects, which are unable to synthesize sterols and must obtain them from their food, seem to have diverted the first part of the metabolic way of sterols to the profit of the biosynthesis of the youthful hormone.

The way of biosynthesis (very simplified) of the JH3 is: acetate (Acétyl-coenzyme has) - > mévalonate - > farnesol - > farnésoate - > methyl-farnésoate - > JH3.

There exists a control of the activity of the bodies allates by Neuropeptide S, is Allatotropine S (which stimulate the activity of biosynthesis of the JH by the bodies allates), that is to say Allatostatine S (which inhibit this activity).

The degradation of the youthful hormone puts primarily concerned enzymes which hydrolize the methyl function ester (it is about JH- Estérase S) or which transforms the epoxy function into a form diol (it acts of JH- epoxy - Hydrolase S). These enzymes are particularly active at the time when the insect enters in phase of metamorphosis.

Proportionings of the youthful hormone (carried out in particular by biological tests, but also by immunological methods of type ELISA or by Gas chromatography) show indeed that the hormonal rates, which are high in the young larvae, but also in the adults, their levels reach low before and during the metamorphosis.

Biological roles of the youthful hormone

Role in the development

Many experiments showed that the youthful hormone maintained the characters larval at the time of the driven and that on the contrary, the absence of this hormone supported the metamorphosis. It is Sir Vincent Wigglesworth which discovered the role of the youthful hormone by experiments on a Punaise Hématophage, Rhodnius prolixus . It thus showed that the Hémolymphe of a young larva was able to delay the metamorphosis of an older larva. On the other hand, the ablation of the bodies allates, source of the youthful hormone, leads the animal to carry out an early metamorphosis, giving an adult of size smaller than the normal. Other experiments were also made by this author on the Ver of flour, '' Tenebrio molitor '', which gave its name to a very significant method of proportioning of the biological activity of the youthful hormone, the Tenebrio - test.

The effects of the youthful hormone on the development are expressed only in the presence of the hormone of moult or Ecdysone, of which it modulates the effects: the ecdysone (or more exactly its active derivative, the 20-hydroxy-ecdysone) starts a driven larval in the presence of youthful hormone, and involves a driven of metamorphosis in its absence.

Role in the reproduction

The most known role of the youthful hormone during the reproduction is the control of the Vitellogenèse. In the majority of the species of insects (except the Dipterous S superiors), this hormone induces the synthesis of Vitellogénine S in the greasy substance of the insect: these vitellogénines is secreted in the hemolymph then absorptive by the ovary, and stored in eggs to constitute the Vitellus. At the dipterous superiors, it is rather the ecdysone which has this function, but the youthful hormone intervenes nevertheless in a synergistic way.

Mode of action

The mode of action of the youthful hormone is not yet known. Does it act via a membrane Récepteur? Or of a Receiving nuclear power? Various authors gave arguments for each of the two assumptions (which are not exclusive besides, as that was shown for other hormones). The protein Usp was proposed like possible nuclear receiver of the youthful hormone, but that remains prone to debate. Let us recall that Usp is the partner of EcR for the constitution of the Récepteur of the ecdysone.

Juvénoïdes and their insecticidal use

One owes the discovery of the insecticidal role of derived from the youthful hormone with a Czech researcher, Karel Slama, which, by carrying out a training course in the United States, realized that the filter paper used in the American laboratory disturbed the development of the insects which he studied, a known bug of the kind Pyrrhocoris under the name of gendarme, which however developed perfectly on European paper. The factor paper , coming from American pines, was insulated and appeared very near to the structure of the youthful hormone. It was thus the first made up juvénoïde, i.e. a Modulateur endocrinien having mimetic properties of the youthful hormone, and disturbing the development of the insects. Since, many others made up neighbors were isolated from plants then, especially, synthesized by the methods of the Organic chemistry. Some are currently exploited like insecticides in agriculture.

Do the youthful hormones exist in the other arthropods?

The youthful hormones of insects were not found at the others Arthropode S. However, some of them, in particular of the Crustacé S or of the Tique S, are sensitive to made up the juvénoïdes which disturb the development or the reproduction of the insects. For example, at the Lobster, the application of juvénoïdes on young larvae, produced additional larval stages, and thus delays the metamorphosis, as in the insect.

It is thus probable that youthful hormones different from the JH of insects exist in the other arthropods. In shellfish, several studies suggest that the methyl-farnésoate (MF) could thus play the youthful part of hormone.

See too

• Moults of the arthropods
• Ecdysone

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