Pheromones isoprenoid pathways

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... 

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... 

Producte of normal metabolism, particularly those of the fatty acid and isoprenoid pathways, were modified by a few pheromone gland-specific enzymes to produce the myriad of pheromone molecules. The elegant work of the Roelofs laboratory [21] demonstrated that many of the lepidopteran pheromones could be formed by the appropriate interplay of highly selective chain shortening of fatty acids and a unique delta-11 desaturase enzyme followed by modification of the carboxyl carbon (see Fig. 5). Chain shortening of fatty acids is also involved in producing the queen pheromone in honeybees [69, 70]. 

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. 

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. 

Evidence for de novo synthesis of pheromone components was obtained by showing that labeled acetate and mevalonate were incorporated into ipsdienol by male Ips pini [103,104]. Similarly, labeled acetate and other labeled intermediates were shown to be incorporated into frontalin in a number of Dendroctonus species [105]. Possible precursors to frontalin include 6-methyl-6-hep-ten-2-one, which was incorporated into frontalin by D. ruffipennis [106]. The precursor 6-methyl-6-hepten-2-one also was shown to be converted to bre-vicomin in the bark beetle, Dendroctonus ponderosae [107]. In addition, the expression patterns of HMG-CoA reductase and HMG-CoA synthase are tightly correlated with frontalin production in Dendroctonus jeffreyi [108, 109]. A geranyl diphosphate synthase cDNA from I. pini was also isolated, functionally expressed, and modeled [110]. These data indicate that the de novo isoprenoid biosynthetic pathway is present in bark beetles. A variety of other monoterpene alcohols such as myrcenol, pityol, and sulcitol are probably synthesized through similar pathways [111]... 

It appears that, in beetles, pheromone production is regulated by JH III, despite the variations in biosynthetic pathways. JH apparently regulates pheromone production in beetles that utilize both fatty acid and isoprenoid biosynthetic pathways [8,98]. Environmental and physiological factors will in turn regulate production of JH. The endocrine regulation of pheromone production in the beetles has been best studied with regard to the bark beetles. 

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).

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