Genetics and breeding of soybeans lacking the Kunitz trypsin inhibitor

The major trypsin inhibitor present in soybean [Glycine max (L.) Merrill] seed is the Kunitz trypsin inhibitor or soybean trypsin inhibitor A2 (SBTI-A2). Four forms of SBTI-A2 have been identified in the U.S. soybean germplasm collection. Three of the forms designated Ti(a), Ti(b), and Ti(c) are electrophoretically distinguishable from one another by their different Rf values of 0.79, 0.75, and 0.83, respectively. The three forms are inherited as codominant alleles in a multiple allelic series at a single locus. The fourth form which is the absence of SBTI-A2 is found in P.I. 157440 and P.I. 196168. The allele lacking SBTI-A2 is designated ti and is inherited as a simple recessive to the other three SBTI-A2 forms. Tia is the most common SBTI-A2 allele in the germplasm collection. Ti(b) primarily is found in Japan and Korea. Ti(c) is associated with the Tohoku District, Japan. P.I. 157440 and P.I. 196168 are from Korea. Linkage studies revealed that the Ti and Ap loci are linked by 16.2 +/- 1.5 map units. Three isolines of soybeans lacking the Kunitz trypsin inhibitor, derived by backcrossing P.I. 157440 to three commercial cultivars (Ti(a)Ti(a)), were released to the research community. Results of preliminary feeding trials, with chicks and young pigs, revealed that gain/feed was significantly higher in lines lacking the Kunitz trypsin inhibitor than cultivars containing the inhibitor. However, the gain/feed was lower in both feeds than commercially processed soybean meal.