Factors limiting hybridization between Penstemon spectabilis and Penstemon centranthifolius

Speciation involves the formation of reproductive isolating mechanisms such as a difference in pollinators, incompatibility between pollen tubes and stylar tissue, hybrid seed abortion, or poor growth of hybrid seedlings. We studied reproductive isolating mechanisms in naturally sympatric populations of Penstemon spectabilis Thurber and Penstemon centranthifolius (Benth.) Benth. where F1 hybrids occurred at very low frequency. We compared conspecific crosses, backcrosses, and heterospecific crosses in terms of pollen grain germination, pollen tube growth, fruit set, seed set, and offspring performance. We found several postpollination barriers to hybridization. When P. spectabilis was the ovule parent, the lack of natural hybridization was partially explained by the presence of two isolating factors: reduced pollen tube growth and reduced seed set. When P. centranthifolius was the recipient, the barrier to hybridization was nearly 100% effective and occurred primarily at the stages of pollen grain germination and fruit set. The success of backcrossing was generally intermediate between conspecific and heterospecific crossing. For these two species, it is likely that partial pollinator specificity in addition to strong postpollination reproductive isolating mechanisms contribute to maintaining the species boundary.