Compatibility of crosses between gene pools and evolutionary classes in the common bean (Phaseolus vulgaris L.)

The extent of genetic divergence between the Mesoamerican and Andean gene pools and between evolutionary classes (wild, landrace and bred) in Phaseolus vulgaris was explored. The compatibility of crosses was assessed by the frequency with which pollination was successful and the number of seeds in the resulting pod. Mesoamerican genotypes were more effective as pollinators than Andean genotypes, regardless of the gene pool of the female parent. Though certain individual genotypes were more easily pollinated by genotypes of one or other of the gene pools, there was no consistent tendency for pollinations between genotypes within a gene pool to be more successful than those between gene pools. In crosses among wild and landrace genotypes, the landrace genotypes were more easily pollinated. In those among wild and bred genotypes, the bred genotypes were more effective as pollinators, and the number of seeds per pod was greater in crosses between the evolutionary classes than in those within classes. In crosses among landrace and bred genotypes, those within evolutionary classes had higher success rates than those between classes (47.2% vs. 38.9%), and produced more seeds per pod. However, these results do not represent a major barrier between the evolutionary classes. The hypothesis that dwarf-lethal incompatibility in inter-gene pool crosses is controlled by two complementary dominant genes (the DL-gene hypothesis) was tested by comparing the numbers of normal and abnormal plants in the progeny of three-way crosses, and the ratios obtained were consistent with the hypothesis. It is concluded that there is no biological barrier between the two gene pools and that they are distinct because, by accident of ancestry, they are located in geographically isolated and ecologically distinct areas, with different cultural practices and preferences. Research programmes aimed at transferring desirable traits between the gene pools should therefore be encouraged.