Flies at Milkweed Fruits
Mebs D, Boppré M, Fischer OW, Schneider M, Reuss E, Wagner M, Kauert G (2008) Fruit flies (Diptera: Tephritidae: Dacus spp.) overcome structural, mechanical and chemical defences of milkweeds (Asclepiadaceae). Abstract of a presentation at the 16th European Section Meeting of the International Society on Toxinology, B-Leuven, Sept. 2008
Background: Seeds of certain milkweeds (Asclepiadaceae) develop under excellent protection: species of, e.g., Calotropis possess as a mechanical barrier large balloon-like, air-filled fruits crossed by radial threads. These, as well as the exo- and endocarp, contain latex serving as a mechanical defence because it coagulates and becomes sticky. Thirdly, most tissues contain cardenolides as chemical defence. Nevertheless, several species of Dacus are able to utilize their unripe seeds as larval food. Material and methods: Egg-laying of Dacus fuscatus, D. longistylus and D. siliqualactis were observed and video-recorded in South-Africa, Ghana and Mozambique, respectively. Flies and maggots were stored in methanol, tissues of Gomphocarpus physocarpus, G. fruticosus, Calotropis procera, and Pachycarpus appendiculatus were dried. Methanolic extracts were analyzed for cardenolides by LC-TOF-MS with digitoxigenin as internal standard. DNA was isolated from flies and amplified by PCR using oligonucleotide primers for the detection of the ouabain-binding site of the Na+,K+-ATPase. Nucleotide sequencing of PCR-fragments was performed on the ABI Prism 310 Genetic Analyzer. For morphological studies light and scanning electron microscopy was employed. Results: Maggots develop in the unripe milkweed ovary which represents the only latex-free tissue. Female flies are able to conquer both the structural and the mechanical barriers with the aid of a telescopic ovipositor which enables egg-laying through the latex-bearing exo-, meso- and endocarp directly into the ovary. Thus, the maggots do not come into contact with latex. All plant parts including the seeds contain variable concentrations of cardenolides, particularly calotropin. Cardenolides also occur in extracts of maggots but not of adult flies. The deduced amino acid sequence of the ouabain-binding site of the Na+,K+-ATPase of the flies indicates that the enzyme is sensitive to cardenolides. Conclusions: The complex mix of defensive properties of milkweed fruits aim at both insects and vertebrates. Structural and mechanical barriers may act against non-adapted insects, while the chemical component is protective against vertebrate herbivores. Fruit flies have managed to utilize a competitor- and enemy-free host for their maggots by morphological and behavioural adaptations in the course of egg-laying and seem not to be affected by cardenolides; likely, these are excreted or degraded by the insects.