Genetic variability for seed size of two- and three-parent soybean populations

Specialty soybean [Glycine max (L.) Merr.] cultivars with less than or equal to 80 mg seed(-1) are needed to produce grain for export to Japan for the production of natto, a fermented food. The purpose of this study was to compare three population types for the recovery of lines that would have adequately small seed size for natto. Two small-seeded and one normal-size conventional cultivar or line were used to produce a small-seeded X small-seeded two-parent population, a small-seeded X normal-size two-parent population, and a small-seeded X (small-seeded X normal-size) three-parent population. Five sets of the three population types were developed with different parents in each set. The seed size of 100 random F-2 plants was determined from each of the 15 populations and 10 plants of each parent. For each of the five sets, the progeny of the 100 F-2 plants of each population type were compared as F-2:3 lines with 10 entries of each of the three parents at two Iowa locations. The average percentage of lines with a seed size equal to or smaller than one of the parents in a cross was 90% for the small-seeded X small-seeded populations, 4% for the small-seeded X normal-size populations, and 20% for the three-parent populations. An average of 10% of the lines from the small-seeded X small-seeded populations had significantly smaller seed size than either of the parents, and no transgressive segregation for small seed size was observed in the other two population types. For the development of small-seeded cultivars, small-seeded X small-seeded and three-parent populations would provide lines with acceptable seed size. A small-seeded X normal-size population may provide suitable lines if seed size of the conventional cultivar is sufficiently small and adequate resources are available to select for the limited number of small-seeded segregates.