Reproductive strategies and barriers to hybridization between Tellima grandiflora and Tolmeia menziesii (Saxifragaceae)

We examined the reproductive biology of Tolmeia menziesii and Tellima grandiflora (Saxifragaceae) and barriers to hybridization between these species. Crossing experiments and pollinator observations suggest that Tolmeia is completely self-incompatible, obligately outcrossing, and bumble bee pollinated, whereas Tellima is self-compatible, with a mixed mating system possibly facilitated by rove beetles. Hybrid crosses set seed only when Tellima was the pollen recipient. Observations of pollen style interactions indicate that self-incompatibility in Tolmeia is late-acting and that pollen tube growth is comparable between reciprocal hybrid pollinations. Seven of 40 progeny from Tolmeia x Tellima crosses were isozymically and morphologically intermediate between Tolmeia and Tellima, and were identical to a naturally occurring hybrid. The remainder were identical to Tellima, the maternal parent in successful hybrid crosses, which we attribute to either intraspecific pollen contamination, or agamospermy. Barriers to hybridization between sympatric Tellima and Tolmeia consist of divergence in floral morphology, pollinator relationships, compatibility, and a series of postzygotic isolating mechanisms including hybrid sterility.