Molecular genetic mapping of a high-lysine mutant gene (opaque-16) and the double recessive effect with opaque-2 in maize

The lysin content in maize endosperm protein is considered to be one of the most important traits for determining the nutritional quality of food and feed. Improving the protein quality of the maize kernel depends principally on finding a mutant with a higher lysine content. Two high-lysine mutant lines with opaque endosperm, QCL3024 and QCL3021, were isolated from a self-cross population derived from Robertson's Mutator stocks. The gene controlling this mutation is temporarily termed opaque-16 (o16). In order to illuminate the genetic locus and effect of the o16 gene, two F2:3 populations, one developed from a cross between QCL3024 and QCL3010 (a wild type line) and another from a cross between Qi205 (opaque-2 line) and QCL3021, were created, and F3 seeds from the F2 plants in the two populations were evaluated for lysine content. The distributions of lysine content and tests for their normality indicate that the lysine content in the two populations is regulated by the major gene of o16 and genes of o2 and o16, respectively. Based on two data sets of the linkage maps of the F2 plant marker genotypes and the lysine content of F3 seeds originating from the two F2:3 populations, the o16 gene was located within 5 cM, at either 3 or 2.2 cM from umc1141 in the interval between umc1121 and umc1141 on the long arm of chromosome 8, depending on the recombination rate in the two populations as determined by composite interval mapping. According to the data of the F2:3 population constructed from the o2 and o16 lines, the double recessive mutant effect was analyzed. The average lysine content of the F3 'o2o2o16o16' families identified by the umc1066 and umc1141 markers was approximately 30% higher than that of the F3 o2o2 and 'o16o16' families, respectively. The lysine content of seven F3 families among nine F3 double recessive mutant families showed different increments, with an average increase of some 6% compared with that of the maternal o2 line. The potential application of the o16 mutant for maize high-lysine breeding may be to combine it with the o2 mutant bearing modifier genes, thus obtaining a mutant with much higher lysine content. For the purpose of pyramiding the o16 with o2 genes, the availability of closely linked markers of the o16 and o2 loci will facilitate marker-assisted selection and greatly reduce breeding time and effort.