AOBPreview published online on August 6, 2007
Annals of Botany, doi:10.1093/aob/mcm161
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Variability in Floral Scent in Rewarding and Deceptive Orchids: The Signature of Pollinator-imposed Selection?
1 Plant Ecological Genetics, Institute of Integrative Biology, ETH Zürich, 8092 Zürich, Switzerland
2 Università degli Studi di Napoli ‘Federico II’, Dipartimento delle Scienze Biologiche, Via Foria 223, Napoli I- 80139, Italy
* For correspondence. Present address: Institute of Systematic Botany, University of Zürich, Zollikerstrasse 107, 8008 Zürich, Switzerland. E-mail florian.schiestl{at}systbot.uzh.ch
Received: 15 March 2007 Returned for revision: 29 May 2007 Accepted: 21 June 2007
Background and Aims: A comparative investigation was made of floral scent variation in the closely related, food-rewarding Anacamptis coriophora and the food-deceptive Anacamptis morio in order to identify patterns of variability of odour compounds in the two species and their role in pollinator attraction/avoidance learning.
Methods: Scent was collected from plants in natural populations and samples were analysed via quantitative gas chromatography and mass spectrometry. Combined gas chromatography and electroantennographic detection was used to identify compounds that are detected by the pollinators. Experimental reduction of scent variability was performed in the field with plots of A. morio plants supplemented with a uniform amount of anisaldehyde.
Key Results: Both orchid species emitted complex odour bouquets. In A. coriophora the two main benzenoid compounds, hydroquinone dimethyl ether (1,4-dimethoxybenzene) and anisaldehyde (methoxybenzaldehyde), triggered electrophysiological responses in olfactory neurons of honey-bee and bumble-bee workers. The scent of A. morio, however, was too weak to elicit any electrophysiological responses. The overall variation in scent was significantly lower in the rewarding A. coriophora than in the deceptive A. morio, suggesting pollinator avoidance-learning selecting for high variation in the deceptive species. A. morio flowers supplemented with non-variable scent in plot experiments, however, did not show significantly reduced pollination success.
Conclusions: Whereas in the rewarding A. coriophora stabilizing selection imposed by floral constancy of the pollinators may reduce scent variability, in the deceptive A. morio the emitted scent seems to be too weak to be detected by pollinators and thus its high variability may result from relaxed selection on this floral trait.
Key words: Anacamptis coriophora, Anacamptis morio, food deception, floral odour, GC-EAD, pollination, scentless flowers
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