Imitation of fermenting fruits in beetle-pollinated Calycanthus occidentalis (Calycanthaceae)

Abstract

Calycanthus (Calycanthaceae), a genus for which three species are described, belongs to an early diverging lineage of angiosperms. As far as known, all species have highly specialized sap beetle (Nitidulidae) pollination systems. While the floral scent is likely to play a key role in attracting the beetles to the flowers, its chemistry is not known for any of the species of this genus. Herein, we analyzed the floral scent of the species Calycanthus occidentalis by dynamic headspace and gas chromatography / mass spectrometry (GC/MS), and determined its floral visitors in plants cultivated in the Botanical Garden of the University of Ulm. The protogynous flowers had one-to-two days lasting anthesis, similar to what was previously reported for native stands in California. More than ten pollinating beetle species, mainly Nitidulidae and Staphylinidae, were collected from the floral chambers of pistillate-stage flowers and identified to species level by DNA barcoding. The high number of beetle species inside the flowers was surprising, especially considering that only the nitidulid species Colopterus truncatus was known so far as pollinator in native habitats. In addition, we observed some drosophilid flies on, but not inside the flowers, which suggests that they are not pollinators. The scent of flowers had a strong alcoholic note, especially in the pistillate phase. The GC/MS data revealed that the most prominent scent compounds were ethanol, ethyl acetate, acetoin, 2,3-butanediol isomers, acetoin acetate, 2,3-butandiol monoacetate, and 2,3-butandiol diacetate. Interestingly, besides scent compounds that were previously found in plants pollinated by nitidulid and staphylinid beetles, some other compounds, including acetoin acetate and 2,3-butandiol monoacetate, were observed for the first time as floral scent of plants attracting such beetles. However, further studies will be required to investigate whether the stereoisomeric pattern of these compounds differs among plant species and whether the stereoisomers attract different pollinator species. Overall, the scent of C. occidentalis is similar to that of other plant species that release scents reminiscent of fermenting fruits and are pollinated by beetles and/or drosophilid flies, such as Asimina triloba, Ceropegia crassifolia and Arum palaestinum.