Lu

V-NHp spermidine

CH2OH hydrolysis of ester

monocrotaline N-oxide danaidone male sex attractant seneciphylline N-oxide

Figure 10.4 Some examples ofpyrrolizidine alkaloids and the sex attractants made from them by danaid butterflies and ornate moths. The biosynthesis in plants of the retronecine part of the pyrrolizidine alkaloids begins with reaction between putrescine (from ornithine, Chapter 9) and spermidine monocrotaline N-oxide danaidone male sex attractant seneciphylline N-oxide

Figure 10.4 Some examples ofpyrrolizidine alkaloids and the sex attractants made from them by danaid butterflies and ornate moths. The biosynthesis in plants of the retronecine part of the pyrrolizidine alkaloids begins with reaction between putrescine (from ornithine, Chapter 9) and spermidine

Some adapted insects can metabolize the alkaloids and use the metabolites as pheromones. For example, male danaid butterflies collect pyrrolidine alkaloids from Senecio plants, part of the alkaloid they store, and part degrade to danaidone (Figure 10.4) to use as a sex attractant. The danaidone encourages the females to copulate with them. It shows the females how rich in alkaloids the males are. The danaidone is presented on organs called hairpencils. Female butterflies are able to detect which males have more toxin by the amount of danaidone they secrete, and choose males with most alkaloid. The males pass on the alkaloids with their sperm, and the females invest their eggs with the alkaloids in turn.

The ornate moth Utethesia ornatrix produces (i?)-(-)-hydroxydanaidal (Figure 10.4) from pyrrolizidine alkaloids like monocrotaline N-oxide, and uses it as a sex attractant for females as the danaid butterflies do. The aphid Aphis jacobaeae also feeds on Senecio plants and stores these alkaloids. The ladybird Coccinella septempunctata (Plate 3) feeding on the aphids accumulates the pyrrolizidine alkaloids in turn, but does not accumulate cardiac glycosides (see earlier).

Two groups of chrysomelid beetles have evolved different ways of dealing with these alkaloids. The genus Oreina feed on Senecio and Adenos-tyles plants. They make their own cardiac glycosides (see Figure 7.12), and accumulate pyrrolizidines as N-oxides, in their haemolymph and glands. One species has only the pyrrolizidines for its defence. Generally the insects do not alter the alkaloids, but two examples, one of hydrolysis and one of epoxidation are known. A typical Oreina alkaloid and these two metabolized products are shown in Figure 10.5.

The mechanism for handling alkaloids in the genus Platyphora is different. Only open-chain pyrrolizidines are accumulated, along with the saponins these insects make themselves (Figure 7.13). The alkaloids are stored as the tertiary amines, not the N-oxides, and only in the haemolymph of larvae and defensive glands of adults. Some alteration of their structures is possible by the insects as shown in Figure 10.6. These species

Figure 10.5 Small changes made in the pyrrolizidine alkaloid acetylseneciphylline N-oxide by Oreina species. The epoxide example is not found in plants
Figure 10.6 Changes made in open-chain pyrrolizidine alkaloid by Platyphora boucardi by epimerization, and new alkaloids made by this species from retronecine

were also shown to be able to take ingested retronecine and esterify it with propionic, lactic and a-hydroxyisovaleric acids to make new alkaloids.

It has been suggested that the toxic effect of pyrrolizidine alkaloids does not really protect insects, because the toxicity only comes into effect when the compound is metabolized, which is well separated from the eating or tasting of the insect, rather it is only the bitterness of the taste of the compounds that offers protection.

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