Mutualism can play a major role in generating biodiversity and in moderating ecosystem functions.  Comparative studies of morphological traits known to mediate mutualistic interactions allow us to identify conditions in which mutualisms are gained and lost.  Here we investigate the evolutionary ecology of two common plant traits in Viburnum species: extrafloral nectaries (EFN), plant organs that secrete small volumes of nectar, thereby attracting ants and other predatory insects to leaves, and leaf domatia, small structures on the undersides of leaves that provide housing for predacious or fungivorous mites.  We test whether large-scale evolutionary patterns support adaptive hypotheses surrounding the taxonomic distribution of these traits in 102 species of Viburnum using maximum likelihood phylogenetic methods.
Results/Conclusions

EFN and domatia occurred on 55% and 58% of the species, respectively.  Both EFN and domatia originated and were lost independently multiple times, although EFN were more conserved than domatia.  For both traits, we categorized different morphological types and found evidence for a sequential evolution of the mutualistic characters.   Finally, although these two mutualistic plant traits occur together on many Viburnums, we found little evidence for correlated evolution.  Understanding the history of mutualistic traits such as EFN and domatia enhance our understanding of how community-wide species interactions evolve.