Molecular evidence for an Andean origin and a secondary gene pool for the Lima bean (Phaseolus lunatus L..) using chloroplast DNA

Chloroplast DNA (cpDNA) diversity has been examined using PCR-RFLP and RFLP strategies for phylogenetic studies in the genus Phaseolus. Twenty-two spe cies, including 4 of the 5 cultivated species (P. lunatus L., the Lima bean ; P. vulgaris L., the common bean; P. coccineus L., the runner bean and P, polyanthus Greenman, the year-bean), represented by 86 accessions were incl uded in the study. Six PCR primers designed from cpDNA and a total cpDNA pr obe were used for generating markers. Phylogenetic reconstruction using bot h Wagner parsimony and the neighbor-joining method was applied to the restr iction fragment data obtained from each of the molecular approaches. P. coc cineus L. was shown to separate with several species of largely Mesoamerica n distribution, including P. coccineus L. and P. polyanthus Greenman, where as P. lunatus L. forms a complex with 3 Andean species (P. pachyrrhizoides Harms, P. augusti Harms and P. bolivianus Piper) co-evolving with a set of companion species with a Mesoamerican distribution. Andean forms of the Lim a bean are found to be more closely related to the 3 Andean wild species th an its Mesoamerican forms. An Andean origin of the Lima bean and a double d erivative process during the evolution of P. lunatus are suggested. The 3 A ndean species are proposed to constitute the secondary gene pool of P. luna tus, while its companion allies of Mesoamerican distribution can be conside red as members of its tertiary gene pool. On the basis of these data, an ov erview on the evolution of the genus Phaseolus is also discussed.