Genetic Relationships within Old U.S. Soybean Cultivar Groups

Before hybridization programs began in the 1930s, new soybean cultivars in the USA were primarily plant introductions from Japan, China, and Korea, or “off type, pure line” selections from existing U.S. cultivars. These “off type, pure‐line” selections were sometimes thought to have arisen as mutations. Current study of this hypothesis is difficult because numerous accessions with names similar or identical to“pre ‐1930” cultivars occur in the USDA soybean germplasm collection. This study was conducted to determine if selections made from old cultivars were likely derived from mutation and to determine the genetic diversity between accessions with the same or similar old cultivar names. One hundred‐six old entries, which belonged to 17 cultivar groups, were analyzed at between 37 and 50 RFLP loci. “Group” was defined as all accessions with a similar common name, and subsequent selections made from them. Genetic diversity values suggested that mutation did not appear to play a key role in the derivation of selections from most old cultivars. Mutation may have contributed to selections made from Habaro, Mandarin, and Wilson, but this could not be determined unambiguously. AK was previously known to be a heterogeneous seed mixture, and the old cultivar Manchu was suspected as such. The RFLP analysis supported the hypothesis that both AK and Manchu were introduced as a heterogeneous line. Most old cultivars with the same or similar common names could be easily distinguished by genotypic (RFLP) or phenotypic diversity. In 44 instances, phenotypic diversity values between two members of a group equalled zero. In only four cases were genotypic diversity values unable to resolve differences between the two accessions.