Juvenile hormone

Juvenile hormones (JHs) are a group of acyclic sesquiterpenoids that regulate many aspects of insect physiology, such as development, reproduction, diapause, and polyphenisms. ^{[1] [2] [3]} In insects, JH (also neotenin) refers to a group of hormones which ensure growth of the larva, while preventing metamorphosis. Because of their rigid exoskeleton, insects can grow only by periodically shedding their exoskeleton - called molting.

Juvenile hormones (JH) 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. There are several different types of JH. 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. ^[4] Certain species of crustaceans have been shown to produce and secrete methyl farnesoate, which is juvenile hormone III lacking the epoxide group. ^[5] Methyl farnesoate is believed to play a role similar to that of JH in crustaceans.

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Control of development

The titre of JH found in the haemolymph of the developing insect controls the stage of development that the insect is in. During ecdysis the form of the new cuticle laid down before the next moult is controlled by the JH level in the insect. JH maintains a juvenile state, and so the level of it gradually decreases during the development of the insect, allowing it to proceed to successive instars with each moult.

This has been demonstrated in various studies, most prominently that by V. B. Wigglesworth in the 1960's. In this study, two adult Rhodnius had their blood systems linked, ensuring that the JH titre in both would be equal. One was a third instar Rhodnius, the other was a fourth instar. When the corpora allata of the third instar insect were removed, the level of JH was equal in both insects to that in the fourth instar animal, and hence both proceeded to the fifth instar at the next moult. When the fourth instar Rhodnius had its corpora allata removed, both contained a third instar level of JH and hence one proceeded to instar four, and the other remained at this instar.

Generally, the removal of the corpora allata from juveniles will result in a diminutive adult at the next moult. Implantation of corpora allata into last larval instars will boost JH levels and hence produce a supernumary (extra) juvenile instar.

Juvenile hormones in honey bees

There is a complex interaction between JH, the hormone ecdysone and vitellogenin. In the development stage, as long as there is enough JH, the ecdysone promotes larva-to-larva molts. With lower amounts of JH, ecdysone promotes pupation. Complete absence of JH results in formation of the adult. ^[6] In adult honey bees, JH and Vitellogenin titers in general show an inverse pattern. ^{[7] [8] [9] [10]}

JH titers in worker honey bees progressively increase through the first 15 or so days of the worker's life before the onset of foraging. ^[11] During the first 15 days, workers perform tasks inside the hive, such as nursing larvae, constructing comb, and cleaning cells. JH titers peak around day 15; workers this age guard, remove dead bees from the colony, and fan at the colony entrance to cool the nest. Aggressiveness of guard bees is correlated with their blood JH levels. Even though guards have high JH levels, their ovaries are relatively undeveloped. ^{[12] [13]} Although. JH does not activate foraging, but rather is involved in controlling the pace at which bees develop into foragers. ^[14]

Vitellogenin titers are high in the beginning of adult life and slowly decreasing.

JH has been known to be involved in the queen-worker caste differentiation during the larval stage. ^[15] The unique negative relationship between JH and Vg may be important to the understanding of queen longevity. ^[16]

Forms

• Juvenile hormone 0 (found in Lepidoptera)
  • CAS methyl (2E,6E)-10R,11S-(oxiranyl)-3,7-diethyl-11-methyl-2,6-tridecadienoate
  • Formula: C₁₉H₃₂O₃

• Juvenile hormone I (found in Lepidoptera)
  • CAS methyl (2E,6E)-10R,11S-(oxiranyl)-7-ethyl-3,11-dimethyl-2,6-tridecadienoate
  • Formula: C₁₈H₃₀O₃

• Juvenile hormone II (found in Lepidoptera)
  • CAS methyl (2E,6E)-10R,11S-(oxiranyl)-3,7,11-trimethyl-2,6-tridecadienoate
  • Formula: C₁₇H₂₈O₃
• Juvenile hormone III
  • CAS methyl (2E,6E)-10R-(oxiranyl)-3,7,11-trimethyl-2,6-dodecadienoate
  • Formula: C₁₆H₂₆O₃
• Juvenile hormone JHB3 (found in diptera)
  • CAS methyl (2E,6E)-6S,7S,10R-(dioxiranyl)-3,7,11-trimethyl-2,6-dodecadienoate
  • Formula: C₁₆H₂₆O₄

• Methyl farnesoate
  • CAS methyl (2E,6E)-3,7,11-trimethyl-2,6-dodecadienoate
  • Formula: C₁₆H₂₆O₂

Use as an insecticide

Synthetic analogues of the juvenile hormone are used as an insecticide, preventing the larvae from developing into adult insects. At high levels of JH, larva can still molt, but the result will only be a bigger larva, not an adult. Thus the reproductive cycle is broken. One insecticide, methoprene, is approved by WHO for use in drinking water cisterns to control mosquito larvae.

Juvenile Hormone Regulation

Juvenile hormone is produced in the Corpora Allatum of insects. JH will disperse throughout the hemolymph and act on responsive tissues. Degradation of JH by enzymes like Juvenile Hormone Esterase (JHE) or Juvenile Hormone Epoxidhydrolase (JHEH). JHE and JHEH are both JH signal suppressors and JH signal responsive. Tissues responsive to JH, can be identified by the expression or presence of JHE.

Reproductive Roles for Juvenile Hormone

JH stimulates the accessory glands of adult males to promote gland growth and sex peptide production. Yolk production, vitellogenesis in female ovaries is also stimulated by JH action. JH levels in both males and females, to some degree, regulate reproductive behavior as well.

References

1. ^ Riddiford, L. M. (1994) Adv. Insect Physiol. 24, 213–274.
2. ^ Wyatt, G. R. & Davey, K. G. (1996) Adv. Insect Physiol. 26, 1–155
3. ^ Nijhout, H. F. (1994) Insect Hormones (Princeton Univ. Press, Princeton)
4. ^ Richard D.S., Applebaum S.W., Sliter T.J., Baker F.C., Schooley D.A., Reuter C.C., Henrich V.C., Gilbert L.I. (1989) Juvenile Hormone Bisepoxide Biosynthesis in vitro by the Ring Gland of Drosophila melanogaster: A Putative Juvenile Hormone in the Higher Diptera. Proc Natl Acad Sci USA 86: 1421-1425
5. ^ Laufer, H, Borst, D, Baker, FC, Carasco, C, Sinkus, M, Reuter, CC, Tsai, LW, and Schooley, DA. (1987) Identification of a juvenile hormone-like compound in a crustacean. Science 235: 202-205
6. ^ Kimball, John W. (2002) Insect hormones
7. ^ Hartfelder K, Engels W (1998) Curr Top Dev Biol 40:45–77
8. ^ Bloch G, Wheeler DE, Robinson GE (2002) in Hormones, Brain, and Behavior, ed Pfaff D (Academic, New York) Vol 3, 195–236
9. ^ Fluri P, Sabatini AG, Vecchi MA, Wille H (1981) J Apic Res 20:221–225
10. ^ Fahrbach SE, Giray T, Robinson GE (1995) Neurobiol Learn Mem 63:181–191
11. ^ Elekonich, M. M., Schulz, D. J., Bloch, G. and Robinson, G. E. (2001). Juvenile hormone levels in honey bee (Apis mellifera L.) foragers: foraging experience and diurnal variation. J. Insect Physiol. 47,1119 -1125
12. ^ Pearce A.N., Huang Z.Y., Breed M.D. (2001) Juvenile hormone and aggression in honey bees, J. Insect Physiol. 47, 1243– 1247.
13. ^ Breed, Michael D. (2002) Juvenile Hormone
14. ^ Sullivan, J. P., Jassim, O., Fahrbach, S. E. and Robinson, G. E. (2000). Juvenile hormone paces behavioral development in the adult worker honey bee. Horm. Behav. 37, 1-14
15. ^ Rachinsky A., Hartfelder K. (1990) Corpora allata activity, a primer regulating element for caste juvenile hormone titer in honey bee larvae (Apis mellifera carnica), J. Insect Physiol. 36, 189– 194
16. ^ Miguel Corona, Rodrigo A. Velarde, Silvia Remolina, Adrienne Moran-Lauter, Ying Wang, Kimberly A. Hughes, and Gene E. Robinson Vitellogenin, juvenile hormone, insulin signaling, and queen honey bee longevity PNAS, Apr 2007; 104: 7128 - 7133

• Wigglesworth, VB. (1964) The hormonal regulation of growth and reproduction in insects. Adv. Insect Physiol. 2: 247-336
• Wigglesworth, VB. (1939) The Principles of Insect Physiology. Cambridge University Press. Cambridge.
• Berger and Dubrovsky (2005) Juvenile Hormone Molecular Actions and Interactions During Development of Drosophila. Vitamins and Hormones. 73:172-215