Speciation involves the origination of reproductive isolating mechanisms, such as a difference in pollinators, incompatibility between pollen tubes and stylar tissue, or poor growth of hybrid seedlings. We studied reproductive isolating mechanisms in a natural sympatric population of Penstemon spectabilis and Penstemon centranthifolius where natural F1 hybrids occurred at very low frequency. We compared conspecific crosses, backcrosses and heterospecific crosses in terms of pollen germination, pollen tube growth, fruit set, seed set, and seed weight. When P. spectabilis was the recipient, heterospecific crosses had reductions in pollen tube growth, fruit set, seed set, and seed mass. When P. centranthifolius was the recipient, heterospecific crosses had reduced pollen grain germination, and they set nearly no fruit. Backcrosses were generally intermediate between conspecific and heterospecific crosses, at least when differences were significant. In this system, partial pollinator specificity also contributes to maintaining the species boundary, and we are currently quantifying the performance of conspecific, backcross, and F1 offspring.

Key words: hybrids, Penstemon, reproductive isolating mechanisms