Using a method called headspace adsorption, the authors sampled the scents of 39 oil-secreting Coryciinae orchids. Flowering stems for each species were enclosed in a glass vessel, and air was pumped through the vessel forcing scents to pass through a glass capillary tube, after which their distinct chemical compound signatures were captured via gas chromatography.
While 257 compounds from nine different compound classes were identified, each orchid species on average had 26 different compoundsmore than 60% of the compounds were found in only one or two orchid species, and only 3% were found in more than half of the taxa.
Contrary to Steiner et al.'s expectations, in the winter rainfall region, phylogenetics seems to play a significantly greater role than pollinator selection pressure. Here the scent profiles fell along phylogenetic linesrelated taxa had similar scents, while specific bee species pollinated taxa with different scents, and taxa with similar scents were pollinated by different bee species. Similarly, in the summer rainfall region, scent profiles also fell along phylogenetic lines, although the authors could not examine pollinator effects because most of the orchids in this region shared the same pollinators.
Interestingly, almost all the orchids in the two regions emitted similar scent compounds even though the Rediviva bee pollinator species differed between the two regions. This was also contrary to the authors' expectations.
Overall Steiner et al. found overwhelming support for the fact that phylogeny played a more important role in scent variation than pollinator selection, even in a group of closely related plant species pollinated by a single class of pollinator.
"We have shown that although there is evidence for pollinator-mediated selection in the chemical composition of some of the scents we analyzed," concludes Steiner, "for
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American Journal of Botany