Common bean (Phaseolus vulgaris) consists oftwo major geographic gene pools, one distributed in Mexico, Central America, and Colombia and the other in the southern Andes (southern Peru, Bolivia, and Argentina). Amplification and sequencing of members of the multigene family coding for phaseolin, the major seed storage protein ofthe common bean, provide evidence for accumulation of tandem direct repeats in both introns and exons during evolution of the multigene family in this species. The presumed ancestral phaseolin sequences, without tandem repeats, were found in recently discovered but nearly extinct wild common bean populations of Ecuador and northern Peru that are intermediate between the two major gene pools of the species based on geographical and molecular arguments. Our results illustrate the usefulness of tandem direct repeats in establishing the polarity of DNA sequence divergence and therefore in proposing phylogenies.

M13 DNA fingerprinting was used to determine evolutionary changes that occurred in Latin American germ plasm and USA cultivars of commonbean (Phaseolus vulgaris L.) during domestication. Linkage mapping experiments showed that M13-related sequences in the common-bean genome were either located at the distal ends of linkage groups or that they were unlinked to each other or to any previously mapped markers. Levels of polymorphism observed by hybridization with M13 (1 probe-enzyme combination) were comparable to those observed by hybridization with single-copy random PstI genomic probes (36 enzyme-probe combinations) but were higher than those observed for isozymes (10 loci). Results indicated that the wild ancestor had diverged into two taxa, one distributed in Middle America (Mexico, Central America, and Colombia) and the other in the Andes (Peru and Argentina); they also suggested separate domestications in the two areas leading to two cultivated gene pools. Domestication in both areas led to pronounced reductions in diversity in cultivated descendants in Middle America and the Andes. The marked lack of polymorphism within commercial classes of USA cultivars suggests that the dispersal of cultivars from the centers of origin and subsequent breeding of improved cultivars led to high levels of genetic uniformity. To our knowledge, this is the first crop for which this reduction in diversity has been documented with a single type of marker in lineages that span the evolution between wild ancestor and advanced cultivars.

Eighty-five wild and cultivated accessions of common bean (Phaseolus vulgaris L.), representing a wide geographic area in the centres of domestication were tested for restriction fragment length polymorphisms (RFLPs). Genomic DNA was digested with one of three restriction enzymes (EcoRI, EcoRV, and HindIII) and hybridized to 12 probes distributed throughout the common bean genome. Accessions could be classified into two major groups with a distinct geographical distribution in Middle America and the Andes. Within each gene pool, cultivated accessions clustered together with wild forms from the same geographical area supporting the multiple domestications hypothesis for this crop. Estimates of Nei's genetic distances among the cultivated races from the two different gene pools varied from 0.12 to 0.56 and among races from the same gene pool from 0.04 to 0.12, suggesting that the divergence in Phaseolus vulgaris has reached the subspecies level. The level of genetic diversity (Ht = 0.38) was twice the value obtained with isozyme analysis. Genetic diversity within races (Hs = 0.27) was four to five times higher compared with isozymes, but genetic diversity between races (Dst = 0.11) was similar for both categories of markers. These results corroborate previous studies on the characterization of genetic diversity in common bean that clearly showed two distinct gene pools, Middle American and Andean. Moreover, RFLP markers are superior to isozymes because they provide better coverage of the genome and reveal higher level of polymorphisms.