Many hummingbird-pollinated plant species evolved from bee-pollinated ancestors independently in many different habitats in North and South America. The mechanisms leading to these transitions are not completely understood. We conducted pollination and germination experiments and analyzed additional reproductive traits in three sister species pairs of which one species is bee- and the other hummingbird-pollinated. All hummingbird-pollinated species showed higher seed set and germination rates in cross-pollinated than in self-pollinated flowers. In the self-compatible, bee-pollinated sister species this difference did not exist. As expected, seed set and germination rate were higher after cross-pollination in the largely self-incompatible genus Penstemon independently of the pollination syndrome. However, the bird-pollinated species produce only half of the amount of ovules and pollen grains per flower compared to the bee-pollinated sister species. This indicates that hummingbird pollination is much more efficient in self-incompatible populations because hummingbirds waste less pollen and provide higher outcrossing rates. Therefore, hummingbird pollination is less resource costly. Overall, we suggest that hummingbirds may increase the reproductive success compared to bees, influencing the evolution of hummingbird pollination in ecosystems with diverse bee assemblages.
Keywords: bee; germination rate; outcrossing; pollination efficiency; seed set; selfing.
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