Genetic diversity, segregation and recombination in common bean (Phaseolus vulgaris L.)

In a first part, the genetic diversity of 95 representative Chilean common bean (Phaseolus vulgaris L.) landraces was analyzed using eight polymorphic enzyme systems and phaseolin seed protein type. Four of phaseolin types were found in decreasing order of frequency: 'C', 'T', 'S' and 'H. Each type had a specific geographic distribution when the samples where divided into two major areas. Northern and Southern regions. The isozyme analysis also detected a rare polymorphism for Mdh-1 and Mdh-2, a null allele in Dian-2 and polymorphism for Aco-2. Nei's genetic distance based on isozyme diversity indicated that a high percentage of the total variation found in this sample was between landraces and only a small percentage of the variation was detected within populations. Nei's genetic distance analysis clustered the bean landraces into two major groups, each of which had a specific isozyme pattern, seed color a seed size. A principal component analysis of isozyme frequencies showed that only 30 percent of the genotypes analyzed were similar to the Andean genotype check and 9 percent of the samples were similar to the Middle American landrace check suggesting a high frequency of hybrids between gene pools in Chilean landraces. In a second part. Variation for segregation and recombination was analyzed in crosses involving 13 bean parents from different gene pools and races. These parents revealed a moderate level of RFLPs (65 percent) in crosses between Andean x Middle American compared with 16.7 percent in crosses between gene pools. EcoRV and Hindlll digests detected higher levels of polymorphism in crosses between Middle American and Andean genotypes, however EcoRI and Hindlll digests were more effficient in detecting higher levels of RFLP variation in crosses between Andean genotypes. The segregation of a set of RFLP markers belonging to linkage groups D5 and D 78 were analyzed in 11 F2 populations derived from Middle American pour Andean and Andean pour Andean crosses. The analyses showed an average level of 7.1 percent of segregation distortion for RFLPs compared with 28.6 percent for isozyme markers. Preferential transmissions of Middle American alleles were detected in all crosses analyzed between gene pools. The levels of recombination were highly variable among a subset of four of the 11 populations analyzed, which suggests that genetic variation for recombination may exist in common bean.