Effect of the inbreeding depression in progeny fitness of runner bean (Phaseolus coccineus L.) and it is implications for breeding

Moderate levels of selfing despite high inbreeding depression (ID) make runner bean an excellent model for mixed-mating reproductive biology studies in legumes. This work assesses the extent of the ID variation and consistency at different plant growth stages through selfing generations in seven runner bean populations. Field-collected populations after previous isolated multiplication were hand-pollinated in an isolated greenhouse during five generations to produce progeny. Generations were compared for inbreeding effects (δ) on seed germination, survival to flower, and seed weight and yield. The outcrossing rates of the founder populations and the genetic variation and Wright’s ID at the population and generation level were estimated by using 35 polymorphic microsatellite loci. Neutral microsatellite loci were analyzed through generations and populations using different outlier tests to identify loci directly associated with adaptation to inbreeding. Our study revealed patterns of genetic diversity (H _e  = 0.36) and outcrossing rates (ranged from 24 to 44 %) that are consistent with a mixed-mating system. Selfing-pollination procedure significantly affected germination and survival rates, yield, and to a lesser extent seed weight. Three loci had significant hits to genes related to embryonic development when performing BLAST searches to Phytozome database. Results showed a general inconsistency in δ across plant growth stages and populations, suggesting that different deleterious loci are acting at different stages. Inbreeding tended to purge individuals of deleterious recessive alleles to reduce ID. Variation among individuals within populations may lead to the development of inbreeding lineages with lower levels of ID. Several lines that have been self-pollinated for many generations became homozygous at almost all gene loci and produced a uniform population of true breeding progeny and acceptable performance.