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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)
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