Improved Tryptophan Content in Maize with Opaque-2 gene Using Marker Assisted Selection (MAS) in Backcross and Selfing Generations

Abstract

The low nutritive value of the maize endosperm protein is genetically controlled. However, quality protein maize (QPM) which is available today contains the opaque-2 gene (o₂o₂) for double tryptophan content in the endosperm along with numerous modifiers for kernel vitreousness. The
objectives of this study were: 1) to convert normal inbred lines to QPM inbred lines by the backcross method and marker-assisted selection (MAS) of the opaque-2 gene and 2) to estimate the combining ability of QPM inbred lines for yield and some agronomic characters. Crosses were made between normal maize and QPM as female and male parents, respectively. These were backcrossed to the recurrent parent. By the marker phi057, it was shown that all F1 were heterozygous (O₂o₂) while the BC1F1 progenies of Agron20 × Pop65C₆-46 and Agron29 × Pop65C₆-55 had 1:1 ratio of homozygous dominant (O₂O₂) to heterozygous (O₂o₂). The heterozygous offspring were self-pollinated to produce the BC1S1. By the marker of phi057 again, homozygous recessive (o₂o₂) plants were identified in one and six from the crosses of Agron20 × Pop65C₆-46 and Agron29 × Pop65C₆-55, respectively. The protein content in the endosperm of these seven lines ranged from 7.35 to 7.72% and the tryptophan content in the protein
ranged from 0.70 to 0.84% which was in the range known for opaque-2 maize (0.80%), twice more than in normal maize (0.41%). The diallel cross of BC1S2 lines was made by Griffing’s Method 4 (fixed effect) and 21 F1-hybrids were obtained. The best test hybrid yielded 7.67 t ha ¹, which was close to the commercial single cross hybrid (KSX4452; 8.35 t ha^-1). One line (P1) had the best combination of general combining ability effects for grain yield; a second line (P6) was almost equal. Both were considered to be suitable as tester lines for the quality protein maize program.