Abstract
Pollinators are important agents of selection on floral traits, including nectar sugar composition. Although it is widely assumed that the proportion of sugars (mainly sucrose, glucose and fructose) in nectar reflects pollinators’ physiological limitations and digestive efficiency, the relative impact of pollinators and abiotic factors on nectar sugar composition, as well as the generality of these associations across the angiosperms, remain unknown. We compiled data on nectar sugar composition for >1000 plant species, along with information on flower visitors, plant growth form and latitudinal climatic zone, to provide the first comprehensive assessment of correlates of variation in sugar nectar composition in the angiosperms. After assembling a phylogeny linking all species in the dataset, we estimated the amount of phylogenetic signal in the percentage of sucrose and, by applying phylogenetically-informed multiple regressions, we evaluated whether nectar composition was influenced either by the main pollinator group, plant growth form, or latitudinal climatic zone. The relative importance of each of these factors was then assessed through model selection based on Akaike information criteria and deviance partitioning analysis. Nectar was dominated by sucrose in 56.8% of all the species, glucose in 16.7%, and fructose in 5.5%. Nectar in the remaining species was characterized by similar proportions of the three sugars. Variation in the proportion of sucrose was highest (~70%) at the intrafamily level, and had a significant but low phylogenetic signal, which partially reflects phylogenetic conservatism of the pollinator niche. After controlling for phylogenetic effects, the proportion of sucrose was mainly related to pollinator type and secondarily to climate. Accordingly, this study indicates that nectar sugar composition shows high evolutionary lability and its variation reflects plant-pollinator associations.
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