Highly Selective Tuning of a Silkworm Olfactory Receptor to a Key Mulberry Leaf Volatile

Abstract

The olfactory system plays an important role in the recognition of leaf volatiles during the search of folivore insects for a suitable plant host. For example, volatiles emitted by mulberry leaves trigger chemotaxis behavior in the silkworms Bombyx mori, and as a consequence, they preferentially reside on and consume mulberry leaves. Here, we aimed to identify natural chemoattractants and their corresponding olfactory receptors (Ors) involved in silkworm behavior to mulberry leaves. Chemotaxis behavioral assays for headspace volatiles detected by gas chromatography-mass spectroscopy analysis revealed that among the volatiles that were emitted by mulberry leaves, cis-jasmone was the most potent attractant for silkworms, working at a threshold of 30 pg from [corrected] 20 cm distance. Among a total of 66 Ors identified in the B. mori genome, we found that 23 were expressed in the olfactory organs during larval stages. Functional analysis of all the larvae-expressed Ors in Xenopus oocytes revealed that one Or, termed BmOr-56, showed a high sensitivity to cis-jasmone. In addition, the ligand-receptor activity of BmOr-56 reflected the chemotaxis behavioral response of silkworms. We identified cis-jasmone as a potent attractant in mulberry leaves for silkworms and provide evidence that a highly tuned receptor, BmOr-56, may mediate this behavioral attraction. The current study sheds light on the mechanism of the correlation between olfactory perception in folivore insects and chemotaxis behavior to a natural volatile emitted by green leaves.