Host terpenes

Whereas some species oxidize host terpenes more randomly, producing an array of rather unspecific volatiles with little information, others use highly selective enzyme systems for the production of unique olfactory signals. However, apart from transformations of monoterpene hydrocarbons of host trees, oxygenated monoterpenes may well be biosynthesized de novo by the beetles (see below). 

Oxygenated monoterpenes which are found in almost every bark beetle species attacking coniferous trees, include czs-verbenol 246, frans-verbenol 247, and myrtenol 248, representing primary products of allylic oxidation of the host terpene a-pinene 45. Further oxidation of 247 or 248 leads to the... 

Bedard W. D., Tilden P. E., Wood D. L., Silverstein R. M., Brownlee R. G. and Rodin J. O. (1969) Western pine beetle field response to its sex pheromone and a synergistic host terpene, myrcene. Science 164, 1284-1285. 

Some of the aggregation pheromones of scolytid beetles also appear to be synthesized with great chiral specificity. The flight response of both sexes of the western pine beetle Dendroctonus brevicomis to (lR,5S,7R)-(+)-exo-brevicomin (XX), host terpenes, and racemic frontal in (XXI) was much greater than the response when the antipode of brevicomin was substituted (160). Similarly, (lS,5R)-(-)-frontalin was a much more powerful attractant than its antipode when tested in admixture with... 

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. 

As with the previous discussions of the selective forces that mold the communication channel, it is difficult to arrive at either definitive proofs or absolute refutations of these models. Notwithstanding such difficulties, several cases provide illuminating evidence of the potential contribution of sex pheromones to the speciation process. In many of the bark beetles it is known that the aggregation pheromone is biosynthesized from precursors obtained by the feeding adult beetles from the phloem of the host trees. In Ips paraconfusus the pheromone component cw-verbenol is biosynthesized from the host terpene (-)-pinene. Ingestion of the opposite isomer produces the opposite optical form of the pheromone (Renwick et al., 1976). Clearly this suggests that beetles... 

Payne and Dickens (357) have described a technique to elucidate the specificity of the receptor system of the southern pine beetle, D. frontalis. The technique employs the differential adaptation of the antennal olfactory receptors to various test compounds either the single unit recording technique or the electroantennogram (EAG) technique is used. It is designed to determine whether different compounds are recognized by the same receptor site and is based on the exposure of the antennal preparation to one compound until the site is completely adapted, followed by exposure to a test compound. It is claimed that failure to show a response to a test compound after adaptation to another indicates that all the chemorecognition sites for the test compound are occupied by the first compound. As it had been shown that the receptors for both bicyclic ketals and host terpenes respond with equal intensites (358), Dickens and Payne (321) calculated the percent of the acceptors (receptor sites) capable of interacting with the various compounds tested. 

If we assume that electrostatic forces play a significant role in the interaction of these bicyclic ketals at the receptor site, then the affinity of the receptor site for frontalin may be greater than that for either endo-or xo-brevicomin. Steric limitations could then account for differences in behavioral activity between the brevicomins. The affinity of the bicyclic ketals, with their two oxygen atoms, for the same receptor site may be considerably greater than that of host terpenes and trans-WQxbtnoX which only have one Ti-bond system. Quantum calculations, if done on a sufficient number of molecules of known biological activities, may provide us with a deeper insight into the mechanism of interaction and may possibly have predictive value. 

The studies on the synthesis of the terpene-derived pheromones have led to the hypothesis that these compounds are waste products from the detoxification of host terpenes that, as a consequence of the timing and conditions of their production and release, have secondarily been utilized as chemical messengers. The nature of these oxidations, the large quantities of products formed, and the formation of the same products by other insects such as house flies when exposed to the terpenes led to the suggestion that the mixed-function oxidase system in the insect may well be involved. ... 

Aggregation of the southern pine beetle, Dendroctonus frontalis. .. on loblolly pine (Pinus taeda) under beetle attack was not disrupted by aerial application of frontalure, which is a mixture of the attractant pheromone frontalin (195) and the host terpene a-pinene (230). Instead, aerial saturation with the pheromone in a heavily beetle-infested pine forest resulted in a rapid increase in the aggregation of beetles on pine trees undergoing attack (549). In this experiment, a ten hectare section of pine forest including 1.6 hectares of a D. frontalis infestation was treated twice by aircraft with rice seed soaked with frontalure. This formulation released virtually all of the frontalure within 24 hr (45 g/ hectare for the first application and 450 g/hectare for the second). 

An entirely different, in fact opposite, effect on insect reproduction by terpenes occurs with the desert locust. In this case the monoterpenes a-pinene, -pinene, llmonene, and euge-nol evaporating from desert shrubs about to bloom, precipitates synchronised sexual maturation and mating activity in the locusts (14). The spruce budworm is also stimulated to increased fertility levels by host tree monoterpenes (R.G. Cates, personal communication). It is possible that even opposite effects on reproduction in insects could occur depending on the specialisation of the insect species to its environment, the diversity of the biological activities of the compounds, and the high level of complexity of the reproductive processes. 

Optically pure, water-soluble macrocyclic TB tetraacid 104 has been examined as a receptor for terpenes. A H NMR study in an ND4C1/ND3 buffer at pD 9.0 in D20 showed that host 104 binds the isomeric menthols with reasonable selectivity [(-)-menthol Ka — 2.5 + 0.2 x 103M 3], ( + )-menthol Ka — 2.0 + 0.2 x 103 M ... 

Small sedentary grazers such as amphipods appear to select chemically defended seaweeds as host plants since they would otherwise be subject to intense predation by reef fishes however, they do not sequester metabolites as do other selective grazers such as sacoglossans and opistobranch molluscs.105 109 Grazing by amphipods induces increased concentrations of acutilol A acetate and acutilol B in Dictyopteris menstrualis and makes the seaweed less susceptible to attack by other predators 112 the same terpenes acted as antifoulants which prevented the settlement of bryozoan larvae.107 This evidence for multiple roles for algal metabolites may provide an explanation of previously documented differences in chemical composition in Dictyota.113... 

Nonapparent plants were fast growing, short-lived, and occurred unpredictably in space and/or time. Thus, many of these plants would escape detection by the majority of herbivores. Because these plants allocated most of their resources to rapid growth and reproduction, they were believed to be defended by relatively low concentrations of toxic compounds that were generally effective against many herbivores but that some specialist herbivores would be able to evolve detoxification mechanisms against (and may even use the qualitative defenses as cues to locate their host). These toxic compounds such as alkaloids and terpenes were termed qualitative defenses because their potency made them effective at low concentrations qualitative defenses of the PAM are analogous to mobile defenses of the RAM. 

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