Sequestration, maintenance, and tissue distribution of pyrrolizidine alkaloid N-oxides in larvae of two Oreina species

Oreina cacaliae and O. speciosissima are leaf beetles that, as larvae and a dults, sequester pyrrolizidine alkaloid N-oxides (PAs) as defensive compoun ds from their host plants Adenostyles alliariae and Senecio nemorensis. As in most Oreina species, O. speciosissima is also defended by autogenously p roduced cardenolides (mixed defensive strategy), whereas O. cacaliae does n ot synthesize cardenolides and is exclusively dependent on host-plant-acqui red PAs (host-derived defense). adults of the two Oreina species were found to have the same PA storage capacity. The larvae, however, differ; larvae of O. speciosissima possess a significantly lower capability to store PAs t han O. cacaliae. The ability of O,Oreina larvae to sequester PAs was studie d by using tracer techniques with C-14-labeled senecionine N-oxide. Larvae of the two species efficiently take up [C-14]senecionine N-oxide from their food plants and store the alkaloid as N-oxide. In O. cacaliae, there is a slow but continuous loss of labeled senecionine N-oxide. This effect may re flect the equilibrium between continuous PA uptake and excretion, resulting in a time-dependent tracer dilution. No noticeable loss of labeled alkaloi d is associated with molting. Senecionine N-oxide is detectable in all tiss ues. The hemolymph is, with ca. 50-60% of total PAs, the major storage comp artment, followed by the integument, with ca 30%. The alkaloid concentratio n in the hemolymph is approximately sixfold higher than in the solid tissue s. The selectivity of PA sequestration in larvae is comparable to PA seques tration in the bodies of adult beetles.