Mayolenes: labile defensive lipids from the glandular hairs of a caterpillar (Pieris rapae)

Abstract

Larvae of the European cabbage butterfly, Pieris rapae (Pieridae), are beset with glandular hairs, bearing droplets of a clear oily secretion at their tip. The fluid consists primarily of a series of chemically labile, unsaturated lipids, the mayolenes, which are derived from 11-hydroxylinolenic acid. In bioassays with the ant Crematogaster lineolata, the secretion was shown to be potently deterrent, indicating that the fluid plays a defensive role in nature.

Figure 1

P. rapae larva (bar = 2 mm).

Figure 2

(A) Detail of dorsum of a P. rapae larva, showing three droplet-bearing glandular hairs. (B) Same as preceding, but rinsed with dichloromethane; glandular hair (arrow) is seen to be secretion free. (C) Enlarged view of tip of hair denoted by arrow in B. (D) Crematogaster ant inspecting a P. rapae larva. (E) Ant, wiping right antenna with foreleg after contacting a P. rapae larva with that antenna. (F) Ant, cleansing its right foreleg with the mouthparts after using that leg to groom an antenna that had touched a P. rapae larva. Bars: A and B = 0.1 mm; C = 5 μm; D = 1 mm.

Figure 3

Bioassay 1: Predation behavior of Crematogaster ant vis à vis P. rapae larvae and mealworms (Tenebrio larvae). (A) Exposure to the P. rapae larva results in increased self-cleaning on the part of the ant (Wilcoxon signed rank, Z = −3.413, P = 0.0006*), whereas exposure to the mealworm results in decreased cleansing (Wilcoxon signed rank, Z = −2.844, P = 0.0045*). (B) The ant makes but brief contact with the P. rapae larva, but extensive contact with the mealworm (Mann–Whitney U, Z = −4.824, P < 0 0001*). For A and B, n =16 in all cases. Bioassay 2: Predation behavior of Crematogaster ant vis à vis P. rapae larvae with intact or experimentally removed secretory droplets. (C) Exposure to the former larvae results in increased cleansing (Wilcoxon signed rank, Z = −2.845, P = 0.0044*), whereas exposure to the latter has no such effect (Wilcoxon signed rank, Z = −0.968, P = 0.33). (D) The ant makes brief contact with the secretion-bearing larvae, but substantially more extensive contact with the secretion-free larvae (Mann–Whitney U, Z = −3.447, P = 0.0006*). For C and D, n = 11 in all cases. P values accompanied by an asterisk are significant at an experiment-wide α = 0.05, as determined by the sequential Bonferonni method. Data are presented as mean ± 1 SE.

Figure 4

Structure of the mayolenes (1), and structures of tetra-unsaturated fatty acids 2 and 3 resulting from intramolecular elimination in the mayolenes.

Figure 5

Derivatization of the natural mixture of mayolenes (1).

Figure 6

Assignment of absolute configuration for the chiral center at C-11 in the mayolenes. Characteristic region of the ¹H-NMR spectra (500 MHz, C₆D₆) of (A) (11R)-methyl-11-[{(1′S,2′R,5′S)-(−)-menthoxydiphenylsilyl}oxy]octadeca-9(Z),12(Z),15(Z)-trienoate, (B) (11S)-methyl-11-[{(1′S,2′R,5′S)-(−)-menthoxydiphenylsilyl}oxy] octadeca-9(Z),12(Z),15(Z)-trienoate, and (C) methyl-11-[{(1′S,2′R,5′S)-(−)-menthoxydiphenylsilyl}oxy]octadeca-9(Z),12(Z),15(Z)-trienoate derived from hydrolysis and derivatization of natural material.

Figure 7

Structures of 11-hydroxylinolenic acid (6) and volicitin (7).