Tests of pre- and postpollination barriers to hybridization between sympatric species of Ipomopsis (Polemoniaceae)

The Ipomopsis aggregata species complex (Polemoniaceae) includes species pa irs that hybridize readily in nature as well as pairs that meet along conta ct zones with no apparent hybridization. Artificial hybrids can be made bet ween I. aggregata and I. arizonica, yet morphological intermediates between these two species have not been observed in natural populations. This appa rent lack of hybridization is perplexing given that plants of the two speci es often grow within a few metres of each other and both species have red f lowers visited by the same species of hummingbirds. We used trained humming birds to examine pollen transfer within and between species. We also hand-p ollinated flowers to examine paternal success of heterospecific and conspec ific pollen, testing paternity with electrophoretic examination of seeds. H ummingbirds were not simply better at transferring pollen within than betwe en species. Instead, I. arizonica was a better pollen donor so that conside rable pollen transfer was observed from I. arizonica to I. aggregata, but v ery little in the opposite direction. Conversely, once pollen arrived at st igmas. I. arizonica pollen performed very poorly on I. aggregata pistils. H owever, pollen from I. aggregata could, in some cases, sire seeds on I. ari zonica. We hypothesize that hybrids are scarce in nature, in part, because of asymmetric barriers to reproduction: little pollen transfer in one direc tion and poor pollen performance in the other.