Submitted to: Journal of Chemical Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2002
Publication Date: 3/31/2003
Citation: Nitao, J.K., Berhow, M., Duval, S.M., Weisleder, D., Vaughn, S.F., Zanger, A., Berenbaum, M.R. 2003. Characterization of furanocoumarin metabolites in the parsnip webworm, Depressaria pastinacella. Journal of Chemical Ecology. 29(3):671-682.
Interpretive Summary: Certain insects are able to consume plant tissues that contain natural toxins and not be affected by these compounds. The parsnip webworm is able to feed on wild parsnip and is not harmed by the toxins produced by this plant. We have determined that the webworm is able to break down these toxins during the digestive process, by converting them to less reactive forms which we have identified in this research. This will allow us to develop a better understanding of how insects are able to adapt to plant-produced toxins. This information can be used for further research on control of insect pests and to enhance the biological control of wild parsnip, an invasive weed, by the parsnip webworm.
Technical Abstract: Although metabolites of furanocoumarins have been characterized in a wide range of organisms, to date they have been identified in only a single insect species, Papilio polyxenes. Depressaria pastinacella, the parsnip webworm, like P. polyxenes a specialist on Apiaceae, routinely consumes plant tissues higher in furanocoumarin content than does P. polyxenes and is capable of faster cytochrome P450-mediated detoxification of these compounds. In this study, we characterized metabolites of xanthotoxin, a linear furanocoumarin, and sphondin, an angular furanocoumarin, in midguts and frass of parsnip webworms. Two metabolites were isolated and identified from webworms fed artificial diet containing xanthotoxin. LC-ESI-MS analysis resulted in the determination of a MW of 266 for the compound in the frass and one of the compounds in the midgut; 1 confirmed its structure as 6-(7-hydroxy-8-methoxycoumaryl)-hydroxyacetic acid (HCHA). The second compound from the midgut had a MW of 252 and was identified by 1 and 13 analysis as 6-(7-hydroxy- 8-methoxycoumaryl)-hydroxyethanol) (HMCH). Whereas HCHA has been found in frass of Papilio polyxenes fed xanthotoxin, HMCH has not been reported previously in insects. Although the first step of metabolism of xanthotoxin in webworms as well as P. polyxenes is likely the formation of an epoxide on the furan ring, angular furanocoumarin metabolism in webworms appears to differ. The principal metabolite of sphondin was identified as demethylated sphondin (6 hydroxy-2H-furo by LC-ESI-MS and confirmed by 1 and 13 analyses. That webworms produce metabolites of xanthotoxin in common not only with other Lepidoptera (e.g., HCHA) but with other vertebrates (e.g., HMCH) suggests a remarkable conservatism in the metabolic capabilities of cytochrome P450s and raises the possibility that insects may share other detoxification reactions with vertebrates with respect to toxins in food plants.