Nectar metabolomes contribute to pollination syndromes

‘Pollination syndromes’, are suites of convergent floral signals thought to reflect selection from functional pollinator groups, often characterized by physical features such as floral shape and color, yet floral rewards such as nectar may also reflect selection from pollinators. We asked whether nectar chemistry shows evidence of convergence across functional pollinator groups alongside morphological traits, i.e. a ‘chemical pollination syndrome’.

We used untargeted metabolomics to compare nectar and leaf chemical profiles across 19 bee and hummingbird pollination syndrome species, giving special focus to species in the genus Salvia, and selecting species to maximize switching events between pollination syndromes.

We found that independently derived bird-syndrome nectar showed convergence on nectar traits distinct from bee-syndrome nectar, primarily driven by the composition and concentration of alkaloid profiles. We did not find evidence that nectar chemical traits were explained by ‘passive leaking’ of metabolites from leaves to nectar since metabolite abundances were uncorrelated across tissues and many nectar metabolites were not present in leaves. Nectar and leaf metabolomes were strongly decoupled from phylogenetic relationships within Salvia.

These results suggest that functional pollinator groups may drive the evolution of floral reward chemistry, consistent with our ‘chemical pollination syndrome’ hypothesis and indicative of selection by pollinators, but other mechanisms such as pleiotropic relationships between floral color and nectar chemical traits would be consistent as well. In addition, these results support the notion that nectar chemistry can be decoupled from that of other tissues.