EVIDENCE FOR HYBRIDIZATION OF 2 OLD-WORLD RHAMNUS SPECIES - RHAMNUS-CATHARTICA AND RHAMNUS-UTILIS (RHAMNACEAE) - IN THE NEW-WORLD

A highly variable, fully fertile population of Rhamnus that was first established in Ann Arbor, Michigan, circa 1952 was studied to identify the species present and determine whether some individuals were of hy brid origin. Specimens of four taxa hypothesized as being present in t he population-R. cathartica, R. davurica var. davurica, R. davurica va r. nipponica, and R. utilis - were used to establish reference cluster s using a canonical discriminant analysis. Results of the discriminant analysis suggest that the population is a hybrid swarm and that only R. cathartica and R. utilis were involved in the formation of the hybr ids at the study site. Additional analyses with the computer program H YWIN corroborated the results of the discriminant analysis, enabled ci rcumscription of putative hybrids, and for each hy brid provided a pai r of specimens in the data set that best matches the morphologies of i ts parents. Utilization of both discriminant and HYWIN analyses in stu dies of hybrid swarms appears complementary and effective.