RFLP ANALYSIS OF SOYBEAN BREEDING POPULATIONS .1. GENETIC-STRUCTURE DIFFERENCES DUE TO INBREEDING METHODS

Segregating soybean [Glycine mar (L.) Merr.] breeding populations are frequently inbred by either single-seed descent or by bulking progeny at each generation. During inbreeding, repetitive sampling of individu al genotypes, or natural selection, may decrease the potential for gen etic gain in a population. This is most likely to occur in populations that are inbred by the bulk method. This study examines the genetic s tructure of two soybean populations, each containing 261 segregating l ines. One population was derived from a cross of two elite lines and w as inbred by a multiple seed descent (MSD) strategy that is frequently employed by soybean breeders. A second population was derived from an elite x unadapted genotype cross that was inbred by single-seed desce nt (SSD). The genetic composition of each line and the genetic distanc es among lines were estimated using 100 RFLP markers. The genetic cont ribution of each parent to each population was equivalent regardless o f the inbreeding procedure. No individual in either population obtaine d greater than 75% of its genes from a particular parent. Close geneti c relationships among lines were not observed in the SSD population, b ut were common in the MSD population. Cluster analysis of individual l ines within the MSD population indicated that about 18% of the lines w ere due to repetitive sampling and, therefore, did not represent indep endent lineages. The repetitive sampling appeared to be caused by stoc hastic processes, as opposed to systematic factors such as differentia l fecundity among genotypes. Genetic marker analysis of population str ucture provides a method for directly assessing the effectiveness of i nbreeding strategies.