Pollen transfer by natural hybrids and parental species in an Ipomopsis hybrid zone

Models of hybrid zones differ in their assumptions about the relative fitne sses of hybrids and the parental species. These fitness relationships deter mine the form of selection across the hybrid zone and, along with gene flow , the evolutionary dynamics and eventual outcome of natural hybridization. We measured a component of Fitness, export and receipt of pollen in single pollinator visits, for hybrids between the herbaceous plants Ipomopsis aggr egata and I. tenuituba and for both parental species. In aviary experiments with captive hummingbirds, hybrid flowers outperformed flowers of both par ental species by receiving more pollen on the stigma. Although hummingbirds were more effective at removing pollen from anthers of I. aggregata, hybri d flowers matched both parental species in the amount of pollen exported to stigmas of other flowers. These patterns of pollen transfer led to phenoty pic stabilizing selection, during that stage of the life cycle, for a stigm a position intermediate between that of the two species and to directional selection for exserted anthers. Pollen transfer between the species was hig h, with flowers of I. aggregata exporting pollen equally successfully to co nspecific and I. tenuituba flowers. Although this study showed that natural hybrids enjoy the highest quality of pollinator visits, a previous study f ound that I. aggregata receives the highest quantity of pollinator visits. Thus, the relative fitness of hybrids changes over the life cycle. By combi ning the results of both studies, pollinator-mediated selection in this hyb rid zone is predicted to be strong and directional, with hybrid fitness int ermediate between that of the parental species.