Opuntia prolifera, the Coastal Cholla, is common to the coastal sage scrub community extending from Ventura County, California to El Rosario, Baja California. On the basis of morphological intermediacy, O. prolifera is suspected to have originated through hybridization between O. alcahes and O. cholla, both species of coastal and inland deserts of Baja California and Baja California Sur. For an independent test of this hypothesis, we screened populations of O. prolifera and the putative parents for RAPD banding patterns. In order to exclude other potential parents and to distinguish species-specific RAPD bands we included O. bigelovii, O. ganderi, O. tesajo, and O. wolfii in the screening. The results provide abundant support for the hybrization hypothesis as well as insight into various features of the process. Twenty-nine primers revealed 44 bands shared only between O. prolifera and just one of the two putative parents. Unique bands are rare (=2) in O. prolifera compared with O. alcahes (=19) or O. cholla (=23). Lack of marker asymmetry within and among populations of O. prolifera, which is triploid, is consistent with a single diploid-level hybridization event. Trends in the degree of band sharing between O. prolifera and its putative parents suggest a central Baja California origin of the species.

Key words: cholla, hybridization, Opuntia prolifera, RAPD, speciation