Pathways ipsdienol-biosynthetic

Fig. 4 Proposed biosynthetic pathways for the production of ipsdienol in I. pini and frontalin in D. spp. Both pathways utilize an isoprenoid biosynthetic pathway to produce geranyl diphosphate...

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]... 

Figure 6.13 Examples of the application of normal-phase, radio-HPLC to the analysis of de novo biosynthetic pathways in bark beetles (Scolytidae). Demonstration of sex-specific de novo biosynthesis of ipsenol, ipsdienol, and amitinol through radio-HPLC analysis of pentane extracts of Porapak-trapped volatiles from (A) male and (B) female Ips paraconfusus Lanier feeding for 168 h in Pinus ponderosa and (C) male and (D) female Ips pini (Say) feeding for 168 h in Pinus jeffreyi (Seybold et al., 1995b). Demonstration of sex-specific de novo biosynthesis of frontalin through radio-HPLC analysis of pentane extracts of Porapak-trapped volatiles from (E) male and (F) female...

Fish R. H., Browne L. E., Wood D. L. and Hendry L. B. (1979) Pheromone biosynthetic pathways conversions of deuterium-labelled ipsdienol with sexual and enantioselectivity in Ips paraconfusus. Tetrahedron Lett. 17, 1465-1468. 

Fish R. H., Browne L. E. and Bergot B. J. (1984) Pheromone biosynthetic pathways conversion of ipsdienone to (-)-ipsdienol, a mechanism for enantioselective reduction in the male bark beetle, Ips paraconfusus. J. Chem. Ecol. 10, 1057-1064. 

Figure 2 Possible biosynthetic pathways leading to ipsdienol (T20).

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. 

Bark beetles oxidise toxic monoterpenoid hydrocarbons in their respiratory air, which originate from the resin of the tree they colonise, and thereby circumvent this defense barrier of the tree. Converting these compounds into less toxic secondary metabolites serves another purpose, by providing pheromones to attract insects of the same species, of both sexes. Therefore, an oxygen function is often introduced species-specifically and stereoselectively. European spruce bark beetles oxidise (-)-a-pinene to (S)-cis-verbenol, whereas pine beetles produce (R)-trans-verheno. In the California fivespined engraver Ips paraconfu-sus), myrcene is converted into (S)-ipsdienol and (S)-ipsenol. But Ips pint and Ips paraconfusus are also able to synthesise their pheromones de novo via the classical mevalonate biosynthetic pathway. 

Knowledge of the biosynthesis of bark beetle pheromones has been largely conjectural, based on simple metabolic pathways from host terpenes. However, the biosynthetic routes of the pheromone components of 7. paracon-fusus are now known (Fig. 12.3). Both sexes produced m-verbenol when exposed to (-)ff-pinene (Renwick et al., 1976), and tran -verbenol from (+)a-pinene. When exposed to the vapour of myrcene, ipsdienol and ipsenol appeared in the hindguts of male beetles, but not in females (Hughes, 1974 Hughes and Renwick, 1977 Byers et al., 1979), whereas no pheromone was detected in the guts of beetles not exposed to myrcene. Deuterium-labelling techniques have now confirmed that myrcene is converted in male I. para-confusus to ipsdienol and ipsenol (Hendry et al., 1980). Thus, all three known pheromone components are obtained by simple oxidation of host plant chemicals. 

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