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Monoterpene Variation in Ponderosa Pine Xylem Resin Related to Western Pine Beetle Predation
Kareen B. Sturgeon
Vol. 33, No. 3 (Sep., 1979), pp. 803-814
Published by: Society for the Study of Evolution
Stable URL: http://www.jstor.org/stable/2407647
Page Count: 12
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The first goal of this study was to determine the nature and location of the boundary between the two chemical regions of ponderosa pine which converge in northern California and southern Oregon. The boundary follows the crest of the Cascade Mountains except for a northeastward projection of the west-side region into the east-side region in the area south of Mt. Shasta along the Pit River. This projection of ponderosa pines with resin chemistry intermediate between east- and west-side populations is associated with the presence of other plants common to the Sacramento Valley These findings are compatible with other studies of geographic variation in morphological and growth characteristics of ponderosa pine. The second goal of this study was to determine if I could find any evidence that the western pine beetle had been a selective force in the evolution of ponderosa pine. Specifically, I hoped to demonstrate that directional selection for limonene, which is toxic to the beetle, had resulted from differential predation by the western pine beetle on chemically polymorphic populations of the pines. I found that directional selection for limonene had occurred and that high concentrations of this toxin were superimposed upon combinations of the remaining four monoterpenes which are highly variable among trees. In addition, it appears that the insects prefer trees which contain both low concentrations of the toxin, limonene, and high concentrations of alpha-pinene, a possible precursor to components of the pheromone complex. Furthermore, trees that are in extremely low frequency in populations without history of predation are found in high frequency in populations with a history of predation. These data suggest that the western pine beetle may be exerting a frequency-dependent selection pressure on chemically polymorphic populations of ponderosa pine. Lastly, that part of the range of the western pine beetle in which it is most abundant and most destructive is associated with trees in which the resin chemistry is characterized by high concentrations of myrcene, which the beetle requires to synergize its pheromone, and of high concentrations of beta-pinene, which is least toxic to it. Variation in monoterpene composition within and between populations of ponderosa pine appears to be an effective antispecialist chemical defense strategy. Predation by the western pine beetle appears to be both directional, resulting in the survival of trees with large proportions of limonene, and frequency-dependent, substantially increasing the frequency in attacked populations of trees which are rare in undisturbed populations. Once the beetle has depleted its source of preferred trees it is faced with only a few alternatives: 1) it may move into another area where its preferred trees are common, or 2) it may evolve a tolerance to limonene which is, at least currently, toxic to it. However, the combination of large individual variation among trees in composition of the remaining four monoterpenes with large proportions of the toxin would substantially reduce the probability of selection for strains of beetles capable of detoxifying all of these compounds.
Evolution © 1979 Society for the Study of Evolution