Artificial hybridizations of Schiedea and Alsinidendron (Caryophyllaceae: Alsinoideae), genera endemic to the Hawaiian Islands, were used to study the effects of phylogenetic relationship, genetic divergence, breeding system, and population size on the ability of species to cross and produce fertile F₁ hybrids. Using 17 taxa, all crosses attempted produced vigorous F₁ hybrids, although pollen fertility of hybrids varied substantially. Fertility of F₁ hybrids was positively related to genetic identity measured using allozyme variability. Fertility of hybrids was not predicted by phylogenetic relationships, estimated from morphological and molecular data. Hybrids between species with dimorphic breeding systems had significantly higher pollen fertility than other combinations, although this effect was not significant when population size was controlled in the analysis. The association of dimorphism and ability to produce fertile F₁ hybrids is probably indirect: dimorphic species of Schiedea occur in large populations and have high genetic identities. Although artificial hybrids are easily produced in the greenhouse, natural hybridization in Schiedea and Alsinidendron is limited, apparently because instances of sympatry are rare and autogamy is often found in one of the two species occurring sympatrically.

Communicating Editor: Richard Jensen