Heterosis and inbreeding depression in two soybean single crosses

Heterosis is considered to be of little importance in soybean (Glyclne max L. Merr.) because the crop is produced as "pure-line" cultivars or blends of inbred lines. The Fr generations Holladay/Hutcheson (Cross 1) and Brim/Boggs (Cross 2) were generated by hand pollinations. Inbred generations were generated by bulk selfng. The F-1, F-2, F-3, F-4, and F-5 generations were yield-tested in replicated bordered single row plots in multiple years and locations. The average yield of Cross 1 F, was 16% greater than that of the highest-yielding parent and the average yield of the Cross 2 Fr was 5% greater than the highest-yielding parent. Cross 1 showed significant inbreeding depression when regressed on percentage inbreeding which is clear evidence of dominance for yield. Possible genetic bases for heterosis in soybean include gene complementation or interaction of duplicate favorable loci in repulsion, linked dominant alleles that are inherited as a unit, a greater number of dominant alleles in the F, than either parent separately, multiple dosage-dependant regulatory loci, and/or overdominance. The existence of heterosis should be evidence that superior gene combinations are possible. The magnitude of yield heterosis may be a useful criterion for selection among biparental crosses.