5.5.1 Precursors of aggregation pheromone components

5.5.1 Precursors of aggregation pheromone components

Aggregation pheromone components were first identified in bark beetles from male Ips paraconfusus as a synergistic blend of (S)-(- )-ipsenol, (S)-(+)-ipsdienol, and (4S)-cis-verbenol (Silverstein et al. 1966, 1967; Wood et al. 1968). Several other Ips species were soon discovered to produce and respond to various blends of these compounds (Vité et al., 1972). The similarity of chemical structure between a major host monoterpene, myrcene, and ipsenol and ipsdienol (Figs. 4 and 5) led Hughes (1974) to propose that in Ips myrcene was a precursor of these pheromone components. Exposure of males of I. paraconfusus to myrcene vapor resulted in their production of compounds with GC retention times identical to ipsenol and ipsdienol (Hughes, 1974). Byers et al. (1979) confirmed these identifications using gas chromatography and mass spectrometry (GC-MS) and behavioral assays, and reported the male- specific quantitative relationships between precursor vapor concentration and pheromone products. Hendry et al. (1980) radiolabelled myrcene and established the direct conversion of myrcene to the pheromone components (Fig. 8). Fig. 8. Proposed scheme for the conversion of the host tree compound, myrcene, to the pheromone components (S)-(-)-ipsenol and (S)-(+)-ipsdienol in Ips paraconfusus based on radio-labelling experiments and enantiomers found in the male (Silverstein et al., 1966; Wood et al., 1968; Hughes, 1974; Renwick et al., 1976; Byers et al., 1979; Fish et al., 1979; Hendry et al., 1980; Byers, 1981; Byers and Wood, 1981). Conversion arrows with question marks have not been proven. (R)-(-)-ipsdienol does not accumulate in the hindgut but may occur as an enzyme-bound intermediate. However, contrary to the scheme, the amounts of ipsenone, (S)-(-)-ipsenol and (S)-(+)-ipsdienol in males were not correlated with myrcene titers in the host trees.

Earlier, Hughes (1974) had proposed that ipsdienol was directly converted to ipsenol, since topical application of ipsdienol on males resulted in ipsenol production. This was confirmed by deuterium labelled ipsdienol (64% d) being converted to labelled ipsenol (25% d; Fish et al. 1979). Intermediates such as ipsdienone and ipsenone (Fig. 8) also have been suggested in the biosynthetic pathways (Fish et al., 1979; Byers and Birgersson, 1990).

Another host monoterpene, (-)-alpha-pinene, in the vapor phase is converted to (S)-cis-verbenol in both sexes of I. paraconfusus (Fig. 9, Renwick et al. 1976), and the relationship between vapor concentration and pheromone component production was quantified (Byers 1981a). Fig. 9. Stereoselective conversion of the two enantiomers of the host tree monoterpene, alpha-pinene, to pheromone components (4S)-cis- verbenol in Ips paraconfusus and (4R)-trans-verbenol in D. brevicomis.

The production of (-)-cis-verbenol and (+)-trans- verbenol in I. typographus and I. amitinus from the (-)- and (+)- enantiomers of alpha-pinene was shown to have a similar relationship by Klimetzek and Francke (1980). The ratio of cis-/trans-verbenols produced by I. typographus was consistent with the ratio of enantiomers of alpha-pinene in the host tree, which differed between trees of different regions and genotypes (Klimetzek and Francke, 1980; Lindström et al., 1989).

Based on the above studies, a paradigm was established that I. paraconfusus, and probably most other Ips species, use myrcene and (-)-alpha-pinene in their host tree as precursors to ipsenol and ipsdienol or cis-verbenol, respectively. The evolution of host selection behavior by Ips bark beetles could be influenced by the amounts of alpha-pinene and myrcene in the tree (Elkinton et al. 1980). The variation of monoterpenes among trees within a species (Mirov 1961; Smith 1964, 1967) might allow beetles to have evolved preferences for trees that had large amounts of precursor for use in biosynthesis of aggregation pheromone components. A second hypothesis is that tree genotypes may have evolved through natural selection which are lower in pheromone precursor monoterpenes as a means of resistance to bark beetles (Byers, 1989a). It is still not known whether selection pressure by bark beetle predation is severe enough to cause the evolution of trees with lower amounts of alpha- pinene as a means of resistance.

There appears not to be any coevolution between myrcene precursor from the host and its conversion by Ips paraconfusus to certain pheromone components. This is because males of I. paraconfusus produced almost identical amounts of the pheromone components ipsenol and ipsdienol when feeding in five different host pine species, regardless of the large differences in concentrations of myrcene in the host-tree species (Byers and Birgersson, 1990). In fact, digger pine, P. sabiniana, had so little myrcene that it could not be detected by GC-MS, and it was calculated that a beetle would need to eat at least eight times its weight in toxic and repellent oleoresin in order to have any possibility of obtaining the required amounts of myrcene. Apparently in this case the expected energetic advantages of using a precursor from the host that is structurally similar to the pheromone components are outweighed by the need for the beetle to retain control over the ecologically critical pheromone system. The beetle could maintain control by means of either de novo biosynthesis or use of a simpler host precursor found in all potential hosts.

Streptomycin, a 70S ribosome-active antibiotic, when fed to I. paraconfusus inhibited their ability to produce ipsenol and ipsdienol, but the antibiotic had no affect on alpha-pinene conversion to cis-verbenol (Byers and Wood, 1981b; Hunt and Borden, 1989). However, I. paraconfusus reared axenically (without microorganisms) could still produce reduced amounts of ipsenol and ipsdienol (Conn et al., 1984; Hunt and Borden, 1989). Any symbiotic microorganisms involved in pheromone biosynthesis would have to be transovarially transmitted in order to survive the axenic rearing methods (Hunt and Borden, 1989). Juvenile hormone is also implicated in pheromone biosynthesis: topical application induces ipsenol and ipsdienol production in I. paraconfusus in the absence of external myrcene (Hughes and Renwick, 1977). Other pheromones such as verbenone, exo-brevicomin and frontalin are found in emerged D. brevicomis before landing on the tree D. brevicomis (Byers et al., 1984).

Exposure of Ips duplicatus to myrcene vapor causes the beetle to produce small amounts of its pheromone component ipsdienol but not E-myrcenol, the other component (Schlyter et al., 1992). Both compounds are expected to be made from myrcene (Pierce et al., 1987). Ivarsson et al. (1993) found that the biosynthesis of both E-myrcenol and ipsdienol from mevalonate in I. duplicatus could be blocked by the enzyme inhibitor compactin, but the production of cis-verbenol from alpha-pinene in I. typographus was not affected by the inhibitor. These studies, and those on I. paraconfusus, indicate that the major pathway in nature for biosynthesis of the pheromone components ipsdienol, ipsenol, and E-myrcenol in Ips is de novo from mevalonate.

Byers, J.A. 1995. Host tree chemistry affecting colonization in bark beetles, in R.T. Cardé and W.J. Bell (eds.). Chemical Ecology of Insects 2. Chapman and Hall, New York, pp. 154-213.