The privet tree, Ligustrum obtusifolium (Oleaceae), defends with a strong protein-denaturing activity in leaves caused by oleuropein, an iridoid glycoside, activted by b-glucosidase that makes leaf protein non-nutritive by binding to lysine and decreasing the lysine content. We report the strategies of privet specialists to feed on this plant. High concentrations of glycine (50-150mM) existed in the digestive juice of several privet-specialist Lepidoptera and Hymenoptera insects such as Brahmaea wallichii (Brahmaeidae). Both in vitro experiments and in vivo bioassays using Eri silkworm, Samia ricini, showed that glycine is secreted to neutralize the plant defense. Further, larvae of a privet-specialist butterfly, Artopoetes pryeri (Lycaenidae), had a high concentration (60.812 mM) of GABA, a compound known as a neurotransmitter, instead of glycine. In other specialists, b-alanine was found. GABA, b-alanine and glycine as well as alanine, amines, and ammonium ion inhibited the lysine decrease, indicating that amino residues are responsible for the inhibition. However, the amino acids found in digestive juice were far more effective. The results show the first clear example of convergent evolution of herbivoresf molecular strategies to feed on a particular plant with chemical defense as well as the first example of herbivoresf physiological adaptation to iridoids.
Species 1: Lepidoptera Brahmaeidae Bramaea wallichii
Species 2: Lepidoptera Lycaenidae Artopoetes pryeri
Species 3: Lepidoptera Saturniidae Samia ricini (Eri silkworm)
Keywords: plant-insect interactions, coevolution
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