Résultat de la recherche
Detoxification of pyrrolizidine alkaloids by the harvestman Mitopus morio (Phalangidae) a predator of alkaloid defended leaf beetles
The harvestman Mitopus morio (Phalangidae) is a generalist predator. It is known to prey on larvae of the chrysomelid leaf beetle Oreina cacaliae defended by plant acquired pyrrolizidine alkaloids (PAs). Tracer feeding experiments were performed to determine how harvestmen tolerate protoxic PAs. Minced meat containing either [14C]senecionine or [14C]senecionine N-oxide was fed to M. morio and subsequently feces and bodies were analyzed. Labeled alkaloid N-oxide remained stable and was eliminated almost unaltered with the feces; only 10% was recovered as tertiary PA. In contrast, approximately 80% of labeled tertiary alkaloid (senecionine) ingested with the diet was N-oxidized and eliminated; the remaining 20% consisted of unchanged senecionine and a polar metabolite of unknown structure. Harvestmen process their diet by excreting digestive juice, indicated by bleaching of the meat color. Analysis of the processed diet revealed some N-oxidation of [14C]senecionine, suggesting the gut as the site of N-oxidation. Analysis of the bodies of harvestmen 80 hours after the tracer feeding pulse revealed only trace amounts of the polar metabolite. Neither senecionine nor its N-oxide could be detected in the body extracts. The results are discussed in relation to the strategies of PA adapted insects to avoid accumulation of tertiary PAs in living tissues.
Biochemical strategy of sequestration of pyrrolizidine alkaloids by adults and larvae of chrysomelid leaf beetles
Tracer feeding experiments with 14C-labeled senecionine and senecionine N-oxide were carried out to identify the biochemical mechanisms of pyrrolizidine alkaloid sequestration in the alkaloid-adapted leaf beetle Oreina cacaliae (Chrysomelidae). The taxonomically closely related mint beetle (Chrysolina coerulans) which in its life history never faces pyrrolizidine alkaloids was chosen as a ‘biochemically naive’ control. In C. coerulans ingestion of the two tracers resulted in a transient occurrence of low levels of radioactivity in the hemolymph (1–5% of radioactivity fed). With both tracers, up to 90% of the radioactivity recovered from the hemolymph was senecionine. This indicates reduction of the alkaloid N-oxide in the gut. Adults and larvae of O. cacaliae sequester ingested senecionine N-oxide almost unchanged in their bodies (up to 95% of sequestered total radioactivity), whereas the tertiary alkaloid is converted into a polar metabolite (up to 90% of total sequestered radioactivity). This polar metabolite, which accumulates in the hemolymph and body, was identified by LC/MS analysis as an alkaloid glycoside, most likely senecionine O-glucoside. The following mechanism of alkaloid sequestration in O. cacaliae is suggested to have developed during the evolutionary adaptation of O. cacaliae to its alkaloid containing host plant: (i) suppression of the gut specific reduction of the alkaloid N-oxides, (ii) efficient uptake of the alkaloid N-oxides, and (iii) detoxification of the tertiary alkaloids by O-glucosylation. The biochemical mechanisms of sequestration of pyrrolizidine alkaloid N-oxides in Chysomelidae leaf beetles and Lepidoptera are compared with respect to toxicity, safe storage and defensive role of the alkaloids.
Production of cardenolides in different life stages of the chrysomelid beetle Oreina gloriosa
Cardenolide production in larval, pupal, and adult leaf beetles Oreina gloriosa (Coleoptera: Chrysomelidae) is compared. Significantly different levels of cardenolides were found in the various developmental stages of this species: early and late fourth-instar larvae (14 and 19 nmol, respectively), pupae (10 nmol), 1-day-old imagos (8 nmol). The developmental effects on individual cardenolides were considerable. Whereas fourth-instar larvae and pupae mainly produce diglycosides (periplogenin-3-O-[β-d-xylopyranosyl-(1→4)-β-d-allopyranoside] and digitoxigenin-3-O-[β-d-xylopyranosyl-(1→4)-β-d-allopyranoside]) and their corresponding monoglycosides, the mixture of cardenolides in adult beetles is clearly more complex, containing increased amounts of acetylated mono- and diglycosides. There were at least 13 different cardenolides in extracts of 1-day-old imagos, spanning a broader range of polarity than the highly polar cardenolides found in extracts of the immature beetles. The biological significance of these chemical differences is discussed.