Genetic control of high stearic acid content in seed oil of two soybean mutants

Stearic acid is one of the two saturated fatty acids found in soybean [Glycine max (L.) Merr.] oil, with its content in the seed oil of commercial cultivars averaging 4.0%. Two mutants, KK-2 and M25 with two- and six-fold higher stearic acid contents in the seed oil than cv 'Bay', were identified after X-ray seed irradiation. Our objective was to determine the genetic control of high stearic acid content in these mutants. Reciprocal crosses were made between each mutant and 'Bay', and between the two mutants. No maternal effect for stearic acid content was observed from the analysis of F1 seeds in any of the crosses. Low stearic acid content in 'Bay' was partially dominant to high stearic acid content in KK-2 and M25, and high stearic acid content in KK-2 was partially dominant to high stearic acid content in M25. Cytoplasmic effects were not observed, as demonstrated by the lack of reciprocal cross differences for stearic acid content in our analysis of F2 seeds from F1 plants. The stearic acid content in F2 seeds of KK-2 x 'Bay' and M25 x 'Bay' crosses segregated into three phenotypic classes which satisfactorily fit a 1:2:1 ratio, indicating that high stearic acid content in KK-2 and M25 was controlled by recessive alleles at a single locus. The data for stearic acid content in F2 seeds of the KK-2 x M25 cross satisfactorily fit a 3:9: 1: 3 phenotypic ratio. The F2 segregation ratio and the segregation of F3 seeds from individual F2 plants indicated that KK-2 and M25 have different alleles at different loci for stearic acid content. The alleles in KK-2 and M25 have been designated as st1 and st2, respectively. The stearic acid content (>30.0%) found in the st1st1st2st2 genotype is the highest known to date in soybean, but it was not possible to develop the line with this genotype because the irregular seeds failed to grow into plants after germination. Therefore, tissue culture methods must be developed to perpetuate this genotype.