CHARI, JEANNIE AND PAUL WILSON.* Department of Biology, California State University, Northridge, CA 91330-8303. - Factors limiting hybridization between Penstemon spectabilis and Penstemon centranthifolius.
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
Key words: hybrids, Penstemon, reproductive isolating mechanisms