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Anther Evolution: Pollen Presentation Strategies When Pollinators Differ

1. Department of Zoology, University of Toronto, Toronto, Ontario M5S 3G5, Canada; and Rocky Mountain Biological Laboratory, Crested Butte, Colorado 81224;2. Department of Biology, California State University, Northridge, California 91330;3. Division of Biological Sciences, University of Montana, Missoula, Montana 59812;4. Department of Evolution, Ecology, and Organismal Biology, Ohio State University, Columbus, Ohio 43210

Male‐male competition in plants is thought to exert selection on flower morphology and on the temporal presentation of pollen. Theory suggests that a plant’s pollen dosing strategy should evolve to match the abundance and pollen transfer efficiency of its pollinators. Simultaneous pollen presentation should be favored when pollinators are infrequent or efficient at delivering the pollen they remove, whereas gradual dosing should optimize delivery by frequent and wasteful pollinators. Among Penstemon and Keckiella species, anthers vary in ways that affect pollen release, and the morphology of dried anthers reliably indicates how they dispense pollen. In these genera, hummingbird pollination has evolved repeatedly from hymenopteran pollination. Pollen production does not change with evolutionary shifts between pollinators. We show that after we control for phylogeny, hymenopteran‐adapted species present their pollen more gradually than hummingbird‐adapted relatives. In a species pair that seemed to defy the pattern, the rhythm of anther maturation produced an equivalent dosing effect. These results accord with previous findings that hummingbirds can be more efficient than bees at delivering pollen.