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Like the parsnip webworm, Papilio polyxenes, the black swallowtail, feeds exclusively on plants that contain furanocoumarins, including wild parsnips, and also like the  webworm, it copes with furanocoumarins via cytochrome P450-mediated detoxification.   Early on, it was determined that black swallowtails are differentially susceptible to linear and angular furanocoumarins.  The larvae appear to be unaffected by linear furanocoumarins, which are widely distributed among hostplants, but are adversely affected by the angular furanocoumarin angelicin, which has limited distribution.   This finding suggested an escalation in the coevolutionary arms race on the part of the plant because angular furanocouamarins appear to come on the scene more recently in the evolution of this pathway.


The story is a bit more complicated, however, because xanthotoxin and angelicin are not encountered separately in hostplants that contain both compounds (and, indeed, often other furanocoumarins as well).   The two furanocoumarins typically occur together dissolved in essential oils.  As such, it is more realistic to evaluate the effects of the mixture than the separate effects.    It turns out that when larvae are given foliage treated with a mixture of xanthotoxin and angelicin, their performance in terms of growth, consumption, and efficiency of food utilization are jointly and adversely affected compared to larvae administered an equimolar amount of either furanocoumarin separately.

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This impact can be traced physiologically to the joint effects of furanocoumarins on P450 detoxification.   Metabolism of both furanocoumarins is found to be inhibited when combined in a mixture at a total concentration that is equimolar to either furanocoumarin assayed separately, and the rate of combined metabolism is significantly less than the rate of xanthotoxin metabolism alone.




What looks like bird feces and more?       (answer)