A host-inducible cytochrome P-450 from a host-specific caterpillar: molecular cloning and evolution.

Cytochrome P-450 monooxygenases (P-450s) play a critical role in the detoxification of natural and synthetic toxins in a wide range of organisms. We have isolated and sequenced cDNA clones encoding a P-450, CYP6B1, from larvae of Papilio polyxenes (Lepidoptera: Papilionidae), the black swallowtail butterfly. This P-450, cloned from a herbivorous insect, is highly inducible by xanthotoxin, a secondary metabolite abundant in the host plants of this specialized herbivore. On Northern blots, mRNAs crossreactive with CYP6B1 were detected in three Papilio species that, like the black swallowtail, have high levels of xanthotoxin-metabolic P-450 activity and encounter xanthotoxin or related compounds in their host plants; in contrast, no crossreactive mRNAs were detectable in three papilinid species that never encounter xanthotoxin in their host plants and lack detectable xanthotoxin-metabolic activity. These results provide evidence that new P-450s can arise as herbivores colonize different host plants and support the hypothesis that interactions between herbivores and their toxin-producing host plants have contributed to the diversification of the P-450 superfamily.