An oviposition stimulant binding protein in a butterfly

Abstract

Oviposition is evoked by plant compounds, which are recognized by chemoreceptive organs of insects. The swallowtail butterfly, Atrophaneura alcinous, oviposits its eggs on the host plant, Aristolochia debilis, in the presence of only two stimulating compounds: an alkaloid, aristolochic acid, and a monosaccharide, sequoyitol. In our previous study, a unique protein of 23 kDa [Oviposition stimulant(s) binding protein (OSBP)] was found in the forelegs of female, but not male, A. alcinous. The electrophysiological response of A. alcinous to an extract of A. debilis was depressed by the presence of OSBP antiserum, suggesting that OSBP presumably binds to oviposition stimulant(s). We show here, using a highly sensitive fluorescence micro-binding assay that native OSBP binds to a main oviposition stimulant, aristolochic acid, from its host plant, A. debilis. Three-dimensional molecular modeling studies also gave a reasonable structure for the OSBP / aristolochic acid complex. This is the first report of a native chemoreceptive protein binding to an oviposition stimulant ligand in insects.

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Acknowledgements

We would like to thank T. Wazawa, X.G. Zheng, M. Ishiguro, K. Yoshihara for their collaborations. We would like to thank Drs. Y. Ishii and T. Takanashi for their useful comments. We also thank Drs. R. Nishida, A. Yamanaka and Mr. Y. Tsuchihara for material supplies.

Figures and Tables

Figure 1 Scanning electron micrographs (SEM) of the ventral side of the 4^th and 5^th tarsi Atrophaneura alcinous. (A) Male and (B) female. Scale bar, 100 µm.

Figure 2 Immunohistochemical localization of OSBP in chemosensory organs of A. alcinous. (A) Female chemosensory organs without antiserum against OSBP. (B) Female chemosensory organs with antiserum against OSBP. Arrowheads indicate the cuticle. Green fluorescence indicates OSBPs signals, and auto-fluorescence is shown in orange. Scale bar, 20 µm.

Figure 3 Electrophysiological responses to the plant compounds recorded from tarsal contact chemosensilla in a female A. alcinous. (A) Responses to control solution [containing 5% dimethyl sulfoxide (DMSO) and 50 mM choline chloride]. (B) Responses to the methanolic extracts of Aristolochia debilis. (C) Responses to aristolochic acid. (D) Responses to sequoyitol. (E) Responses to the methanolic extracts of A. debilis after ten minute treatment with antiserum against OSBP.

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