Juvenile Hormone Action

Juvenile hormone has a unique terpenoid structure and is the methyl ester of epoxy farnesoic acid (see also Chapter 3.7). This sesquiterpene exists in at least six different forms (Figure 15). JH III is the most common type and is present in most insects. Five different JHs, JH 0, I, II, III, and 4-methyl-JH I, have been described in lepidopterans. The bis-epox-ide of JH, JH B3, is found along with JH III in higher Diptera (Cusson and Palli, 2000). Various studies show that JH III is the main JH in most insects whereas JH I and II are the principal ones in Lepi-doptera. Biosynthesis of JH proceeds through the mevalonate pathway with acetyl coenzyme A (acetyl CoA) serving as the building block. Propionyl CoA is used wherever ethyl side chains occur. Epoxida-tion and esterification are the terminal steps in the biosynthetic process (Schooley and Baker, 1985; Brindle et al., 1987).

JH secretion by the corpus allatum is regulated by two neurohormones, the allatotropins that stimulate secretion and the allatostatins that inhibit production. Severing the nerve connections or extirpating the neurosecretory cells results in the loss of

1990; Feng et al, 1999). JH being highly lipophilic is made soluble by JH binding proteins (JHB) to facilitate transportation to the target sites as well as protect it from degradation from nonspecific esterases (Goodman, 1990). The receptor rich structure of the corpus allatum from the female cockroach, Diploptera punctata, is illustrated in Figure 16 (Chiang et al., 2002).

JH is perhaps one of the most pleiotropic hormones known and functions in various aspects of metamorphosis, reproduction, and behavior (Riddiford, 1994, 1996; Wyatt and Davey, 1996; Cusson and Palli, 2000; Palli and Retnakaran, 2000; Hiruma, 2003; Riddiford et al., 2003).

The major function of JH is the maintenance of the larval status or the so-called juvenilizing effect. During the last larval instar in lepidoptera there is an absence of JH during the commitment peak of 20E, which results in the reprogramming of metamorphosis towards pupation (Figure 17). In the absence of JH, this ecdysone peak induces the expression of the broad complex gene (BrC or Broad), which is a transcription factor that initiates the larval-pupal transformation through several microRNAs (miRNAs) (Zhou and Riddiford, 2002; Sempere et al., 2003). In adults, JH secretion resumes and is responsible for yolk protein (vitellogenin) synthesis and transport into the ovaries (Wyatt and Davey 1996). In addition, JH is also responsible for adult diapause where the ovaries do not develop due to the absence of JH and this effect

Figure 16 Corpus allatum of female cockroach, Diploptera punctata. (a) N-methyl-D-aspartate subtype of glutamate receptors (NMDAR) in the nerve fibers (green) involved in JH synthesis and nuclei (red) of parenchyma cells counterstained with propidium iodide. (b) Allatostatin-immunoreactive nerve fibers in the gland (depth code with different colors indicates distance of an object from the surface). Scale bar = 50 mm. (Reproduced with permission from Chiang, A.-S., et al., 2002, Insect NMDA receptors mediate juvenile hormone biosynthesis. Proc. Natl Acad. Sci. USA 99, 37-42; © National Academy of Sciences, USA.)

Figure 16 Corpus allatum of female cockroach, Diploptera punctata. (a) N-methyl-D-aspartate subtype of glutamate receptors (NMDAR) in the nerve fibers (green) involved in JH synthesis and nuclei (red) of parenchyma cells counterstained with propidium iodide. (b) Allatostatin-immunoreactive nerve fibers in the gland (depth code with different colors indicates distance of an object from the surface). Scale bar = 50 mm. (Reproduced with permission from Chiang, A.-S., et al., 2002, Insect NMDA receptors mediate juvenile hormone biosynthesis. Proc. Natl Acad. Sci. USA 99, 37-42; © National Academy of Sciences, USA.)

control (Tobe and Stay, 1985; Tobe et al., 1985; Lee et al., 2002). JH is cleared from the hemolymph by JH esterase, which selectively cleaves the methyl ester inactivating the hormone (Wroblewski et al.,

Figure 15 Chemical structure of naturally occurring JHs.
Figure 17 Juvenile hormone and 20-hydroxyecdysone (20E) titers in the various stages of a typical lepidopteran. MP, molt peak of 20E; CP, commitment peak of 20E; RD, reproduction peak of 20E.

can be reversed by applying JH. JH has also been shown to play a role in caste determination, phero-mone production, polyphenism, migration, antifreeze protein production, female sexual behavior, male accessory gland secretion, etc. (Wyatt and Davey, 1996).

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