Limited hybridization across an edaphic disjunction between the gabbro-endemic shrub Ceanothus roderickii (Rhamnaceae) and the soil-generalist Ceanothus cuneatus

• Premise of the study: Hybridization is thought to have played an important role in diversification of the speciose shrub genus Ceanothus; putative hybrid species have been described, and data suggest that intrinsic barriers may not exist among closely related species. However, the extent to which hybridization occurs in the wild is not known, and little is understood about how extrinsic factors such as soil chemistry may influence the process. The present research focuses on the gabbro-endemic C. roderickii and the closely related soil-generalist C. cuneatus. Though the species occur peripatrically, they remain distinct across an edaphic disjunction.• Methods: AFLP was used to quantify hybridization and introgression. Biological data and experiments were used to test for prezygotic isolation. Growth trials were used to test for local adaptation and selection against hybrids.• Key results: Ceanothus cuneatus and C. roderickii were strongly differentiated morphologically and genetically, despite a lack of evidence for prezygotic barriers. Hybrids and back-crosses were present but infrequent. Finally, there was selection against hybrids in nonnative soil.• Conclusions: There is little genetic exchange between the focal species across an edaphic disjunction, despite the absence of prezygotic barriers. This result implies that soil conditions, as well as other extrinsic factors, should be considered as forces that may restrict hybridization and gene flow in Ceanothus, influencing local adaptation and speciation. Findings presented here are significant because they imply that exchange of genetic material between plants may be limited directly by the abiotic environment, rather than by the biology of the plants.