Several species in the North American shrub genus Ceanothus L. are purported to be diploid hybrid-derived species, isolated and stabilized from congeners by geographic barriers. Ceanothus masonii and C. sonomensis are putatively of hybrid origin each derived independently from C. cuneatus var. cuneatus and C. gloriosus var. exaltatus. We tested this hypothesis using allozyme, morphometric data, and sequences from the internal transcribed spacers (ITS) and 5.8S coding region of nuclear ribosomal DNA (nrDNA) and the chloroplast-encoded matK gene. We found all four entities to be closely related genetically. Lack of genetic differentiation made it difficult to evaluate the hybrid origin hypothesis. The alternative hypothesis of primary allopatric divergence is just as reasonable an explanation for our data.

Communicating Editor: Paul Wilson