CHEMICAL PHENOTYPE MATCHING BETWEEN A PLANT AND ITS INSECT HERBIVORE

Two potential outcomes of a coevolutionary interaction are an escalati ng arms race and stable cycling. The general expectation has been that arms races predominate in cases of polygenic inheritance of resistanc e traits and permanent cycling predominates in cases in which resistan ce is controlled by major genes. In the interaction between Depressari a pastinacella, the parsnip webworm, and Pastinaca sativa, the wild pa rsnip, traits for plant resistance to insect herbivory (production of defensive furanocoumarins) as well as traits for herbivore ''virulence '' (ability to metabolize furanocoumarins) are characterized by contin uous heritable variation. Furanocoumarin production in plants and rate s of metabolism in insects were compared among four midwestern populat ions; these traits then were classified into four clusters describing multitrait phenotypes occurring in all or most of the populations. Whe n the frequency of plant phenotypes belonging to each of the clusters is compared with the frequency of the insect phenotypes in each of the clusters across populations, a remarkable degree of frequency matchin g is revealed in three of the populations. That frequencies of phenoty pes vary among populations is consistent with the fact that spatial va riation occurs in the temporal cycling of phenotypes; such processes c ontribute in generating a geographic mosaic in this coevolutionary int eraction on the landscape scale. Comparisons of contemporary plant phe notype distributions with phenotypes of herbarium specimens collected 9-125 years ago from across a similar latitudinal gradient, however, s uggest that for at least one resistance trait-sphondin concentration-i nteractions with webworms have led to escalatory change.