Sequestration of ingested [14C]senecionineN-oxide in the exocrine defensive secretions of chrysomelid beetles

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Abstract
Oreina cacaliae (Chrysomelidae) sequesters in its elytral and pronotal defensive secretion theN-oxides of pyrrolizidine alkaloids (PAN-oxides) from its food plantAdenostyles alliariae (Asteraceae). [14C]SenecionineN-oxide was applied for detailed studies of PAN-oxide sequestration. An average of 11.4% of total radioactivity is taken up by individual beetles which had received [14C]senecionineN-oxide with their food leaves 8 days before. An average of 28.9% of the ingested radioactivity could be recovered from the defensive secretions collected twice, i.e., 5 and 8 days after tracer feeding. The tracer transfer into the secretion seems to be a slow but progressive process as indicated by the high percentage of tracer still recovered from the secretion sampled after 8 days. Chromatographic analysis revealed that [14C]senecionineN-oxide is the only labeled compound in the defensive secretion. Beetles that fed on tertiary [14C]senecionine sequestered only trace amounts of radioactivity (exclusively present as labeled IV-oxide) in their secretions.O. speciosissima, a species also adapted to PA containing food plants, was shown to sequester [14C]senecionineN-oxide with the same efficiency asO. cacaliae. O. bifrons, a specialist feeding onChaerophyllum hirsutum (Apiaceae), rejected PA treated leaf samples already at very low PA concentrations (10 nmol/leaf piece). In bothO. cacaliae andO. speciosissima, [14C]senecionineN-oxide applied by injection into the hemolymph is rapidly transferred into the glands.O. bifrons, not adapted to pyrrolizidine alkaloid containing plants was unable to sequester [14C]-senecionineN- oxide in the secretion but rapidly eliminated the tracer with the frass. Again, only traces of labeled [14C]senecionineN-oxide were found in the defensive secretions of the two PA adapted species if labeled senecionine was injected. It is suggested that the beetles are adapted to theN-oxide form of PAs, similarly as their food plants, and that they lack the ability to efficientlyN-oxidize tertiary PAs. No indication forde novo PA synthesis by the beetles was found in tracer feeding experiments with the biogenetic PA precursor putrescine.