Testcross Evaluation of Soybean Germplasm

Abstract

The F 1 generation of a testcross previously was not used to evaluate soybean [ Glycine max (L.) Merr.] germplasm, primarily because production of large amounts of hybrid seed has been impractical and because evaluation of quantitative traits with one or a few plants does not give reproducible results. Recently, a method for efficiently producing testcross seed was developed by using the male sterility locus, Ms6 , and a genetically linked seedling marker locus, W 1 . The objectives of this research were (i) estimation and comparison heterosis and inbreeding depression values for agronomic traits of testcross lines; and (it) determination of the utility of per se and testcross evaluation for selecting germplasm to be included in an existing cultivar development program. Two testers were used to evaluate six lines in three‐row plots. Significant F 1 and F 2 midparent heterosis and inbreeding depression were observed for nearly all traits, including grain yield. The choice of tester was ascertained to be important in determining parental value of germplasm. Parental lines were evaluated by using a combination of per se performance, heterosis values, and a recently developed testcross statistic, Ti . The Ti value was better than F 1 midparent heterosis for identifying lines suitable for use in direct crosses. F 1 midparent heterosis was better than the Ti value for identifying valuable lines requiring pre‐breeding effort. A combination of F 1 and F 2 midparent heterosis values identified lines likely to have genes with dominance effects; although, because of inbreeding depression, use of F 2 heterosis alone failed to identify valuable lines.