FURANOCOUMARIN METABOLISM IN PAPILIO-POLYXENES - BIOCHEMISTRY, GENETIC-VARIABILITY, AND ECOLOGICAL SIGNIFICANCE

The ubiquitous occurrence of series of biosynthetically related plant secondary compounds within individual species has given rise to the su ggestion that such multiplicity is adaptive; one possible mechanism th at would serve to maintain such within-plant diversity is analog syner gism. In a series of experiments, we provide evidence that synergism m ay account for the presence of multiple structurally related furanocou marins in apiaceous plants. The black swallowtail, Papilio polyxenes, feeds exclusively on plant species containing furanocoumarins. Growth of larvae fed parsley leaves treated with both xanthotoxin and angelic in, two furanocoumarins that co-occur widely in swallowtail hostplants , was significantly slower than that of larvae fed leaves with an equi molar concentration of either xanthotoxin or angelicin. A multivariate combination of growth, food consumption and frass excretion differed significantly between larvae fed leaves treated with both xanthotoxin and angelicin and larvae fed leaves treated with angelicin alone. In a ddition, we measured rates of in vitro cytochrome P450-mediated metabo lism of three furanocoumarins - bergapten, xanthotoxin, and angelicin. While bergapten and xanthotoxin, both linear furanocoumarins, were me tabolized at similar rates (8.07 and 9.86 nmoles/min/g fw caterpillar, respectively), angelicin, an angular furanocoumarin, was metabolized more slowly (2.76 nmoles/min/g fw caterpillar). When all three furanoc oumarins were assayed together, overall rates of metabolism were signi ficantly reduced, suggesting substrate inhibition. Thus, the pattern o f growth of larvae is consistent with the pattern of in vitro metaboli sm and is evidence in support of analog synergism. In a separate exper iment, metabolism of xanthotoxin and angelicin individually and togeth er were compared in six maternal families. Again, angelicin was metabo lized more slowly than xanthotoxin and each furanocoumarin inhibited m etabolism of the other. That significant family effects were found for rates of metabolism and for the ratio of moles of angelicin metaboliz ed for each mole of xanthotoxin metabolized raises the possibility tha t genetic variation exists for the rate and specificity of metabolism and suggests that insect herbivores may be able to adapt to analog syn ergism.