Isoprenoid pheromone biosynthesis

Figure 6.16 Model illustrating interspecific regulatory differences in an early-stage reaction in isoprenoid pheromone biosynthesis between male Ips paraconfusus Lanier and Ips pini (Say). Feeding on host phloem results in synthesis of the full amount of the major pheromone component and full activity of HMG-R for both species. The impact of feeding on HMG-R transcript levels is yet to be determined. Topical treatment of male I. pini with JH III mimics feeding nearly completely in terms of pheromone mass and HMG-R activity. Topical treatment of male I. paraconfusus with JH III does not mimic feeding in terms of pheromone mass or HMG-R activity. Topical treatment of both species with JH III results in significantly enhanced levels of HMG-R transcript. One hypothetical explanation for the interspecific difference is that a second hormone (SH) or factor may be associated with the synthesis, stability, and/or activity of HMG-R in I. paraconfusus.

Wiygul et al. (1982) reported the site of pheromone biosynthesis in male boll weevils as the fat body. To date this is the only insect where the fat body has been proposed as a site for pheromone biosynthesis, and it is an obvious contradiction that in most species of the closely related Scolytidae, isoprenoid pheromone biosynthesis has been reported to occur in the alimentary canal (see section 6.6.2). Wiygul et al. (1982) observed that the level of pheromone production... 

Abstract Pheromones are utilized by many insects in a complex chemical communication system. This review will look at the biosynthesis of sex and aggregation pheromones in the model insects, moths, flies, cockroaches, and beetles. The biosynthetic pathways involve altered pathways of normal metabolism of fatty acids and isoprenoids. Endocrine regulation of the biosynthetic pathways will also be reviewed for the model insects. A neuropeptide named pheromone biosynthesis activating neuropeptide regulates sex pheromone biosynthesis in moths. Juvenile hormone regulates pheromone production in the beetles and cockroaches, while 20-hydroxyecdysone regulates pheromone production in the flies. 

Coleoptera comprise the largest order of insects and accordingly pheromone structures and biochemical pathways are diverse [98, 99]. Beetle pheromone biosynthesis involves fatty acid, amino acid, or isoprenoid types of pathways. In some cases dietary host compounds can be converted to pheromones, but it is becoming apparent that most beetle pheromones are synthesized de novo. 

Bark beetles primarily utilize isoprenoid derived pheromones [100,101] and have been the most studied regarding their biosynthesis [8,98]. Earlier work indicated that the isoprenoid pheromones could be produced by the beetle altering host derived isoprenoids however more recent work indicates that for the most part bark beetles are producing pheromones de novo. The production of isoprenoids follows a pathway outlined in Fig. 4 which is similar to the isoprenoid pathway as it occurs in cholesterol synthesis in mammals. Insects cannot synthesize cholesterol but can synthesize farnesyl pyrophosphate. Insects apparently do not have the ability to cyclize the longer chain isoprenoid compounds into steroids. The key enzymes in the early steps of the isoprenoid... 

Figure 6.10 De novo biosynthesis of isoprenoid pheromone components by bark and ambrosia beetles through the mevalonate biosynthetic pathway. The end products are hemiterpenoid and monoterpenoid pheromone products common throughout the Scolytidae and Platypodidae (Figure 6.9A). The biosynthesis is regulated by juvenile hormone III (JH III), which is a sesquiterpenoid product of the same pathway. The stereochemistry of JH III is indicated as described in Schooley and Baker (1985). Although insects do not biosynthesize sterols de novo, they do produce a variety of derivatives of isopentenyl diphosphate, geranyl diphosphate, and farnesyl diphosphate. Figure adapted from Seybold and Tittiger (2003).

Evidence accumulated for and against the paradigm that bark beetle pheromone biosynthesis involved direct modification of host precursor monoterpenes. For 1. pini, the issue was laid to rest with the demonstration that male tissues incorporate radio-labeled acetate into ipsdienol in a manner consistent with pheromone production. Similar experiments proved the de novo biosynthesis of frontalin, an important isoprenoid-derived semiochemical produced by male Dendroctonus jeffreyi It is probable that other Coleoptera can also synthesize monoterpenes, either as pheromone components " or defensive compounds. Despite the capacity for de novo biosynthesis, plant precursor modification is likely an important source of pheromone components for some species. In these cases, plant chemicals could enter the pheromone biosynthetic pathway at later steps. 

TILLMAN, J.A., LU, F., STAEHLE, L., DONALDSON, Z DWINELL, S.C., TITTIGER, C., HALL, G.M., STORER, A.J., BLOMQUIST, G.J., SEYBOLD, S.J., Juvenile hormone regulates de novo isoprenoid aggregation pheromone biosynthesis in pine bark beetles, Ips spp. (Coleoptera Scolytidae), through transcriptional control of HMG-CoA reductase, J. Chem. Ecol, 2004,30, 2335-2358. 

Our comprehension of how JH III interacts with the biosynthesis of fatty acid-or amino acid-derived scolytid pheromones is limited to what has been learned from several preliminary studies, and in contrast to the isoprenoid pathway there is no information on any endocrine effects on the expression or activity of specific pathway enzymes. Hughes and Renwick (1977a) found that topical application of JH III to newly emerged female D. brevicomis resulted in the production of more than 1 pig of ejco-brevicomin per beetle. Feeding on fresh P. ponderosa logs also stimulated exo-brevicomin production in females as did... 

Seybold S. J., Bohlmann J. and Raffa K. F. (2000) The biosynthesis of coniferophagous bark beetle pheromones and conifer isoprenoids evolutionary perspective and synthesis. Can. Entomol. 132, 697-753. 

Volatile isoprenoids that control insect behavior and development have been reviewed.476 Information on the biosynthesis of terpenoids with special emphasis on beetle pheromones has been compiled by Seybold and... 

MILLER, D.R., GIBSON, K.E., RAFFA, K.F., SEYBOLD, S.J., TEALE, S.A., WOOD, D.L., Geographic variation in response of pine engraver, Ips pini, and associated species to pheromone, lanierone, J. Chem. Ecol, 1997,23, 2013-2031. SEYBOLD, S.J., BOHLMANN, J., RAFFA, K.F., Biosynthesis of coniferophagous bark beetle pheromones and conifer isoprenoids Evolutionary perspective and synthesis. Can. EntomoL, 2000,132, 697-753. 

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