Premise of the study: Over the past 75 Myr, successive groups of plants have entered the "oil bee pollination niche," meaning that they depend on oil-collecting bees for their pollination. The highly dissimilar numbers of plant species and bee species involved in these mutualisms imply evolutionary host switching, asymmetric mutual dependencies, and uncoupled diversification. Among the clades with the best field data on oil bee behavior is the Angelonieae, which we here investigate to better understand the evolutionary time frame of this pollination syndrome.

Methods: We generated nuclear and plastid data matrices for 56% of the Angelonieae species (plus outgroups) and used Bayes-ian methods of molecular clock dating, ancestral state reconstruction, and biogeographic inference.

Key results: We found that Angelonieae have two major clades, Angelonia (including Monopera) and Basistemon, and Mont-tea, Melosperma, and Ourisia.Conclusions: Angelonieae date back to the Uppermost Eocene, ca. 35 (26-47) Myr ago (Ma) and diversified in dry areas of southern South America; they switched from nectar to oil as a reward four or five times over the past 25 Ma. As predicted in a previous non-clock-dated study, dispersal to Australasia dates to the Miocene/Pliocene.