Enhancing introgression of the opaque-2 trait into elite maize lines using simple sequence repeats
To improve on the efficiency of Quality Protein Maize (QPM) breeding in Uganda, the utility of three simple sequence repeats (SSR) markers (phi057, phi 112 and umc1066) in selection and introgression of the opaque-2 (o2) gene was investigated. Genomic DNA of six normal and seven (QPM) lines was analysed using a standard PCR protocol. Polymorphisms were detected in the 0paque-2 locus among all the maize lines when using the SSR primers, phi057 and phill2, while no polymorphism was detected with primer umc1066. To facilitate background selection in the heterozygous BC2 progeny, key phenotypic characters of the recurrent parent ( 136R) were used as additional markers. The SSR marker phi057 was co-dominant while ph-ll2 was dominant. The polymorphic SSR markers correctly predicted the expression of tryptophan in kernels of all QPM inbreds and five of the six non-QPM inbred lines. However, one non-QPM inbred line (PED49B) had a tryptophan content that is characteristic of the o2 mutation (0.090), suggesting that another genetic system may be responsible for the expression of trytophan in this maize line. Phi057 was employed to monitor the introgression of the o2 allele from CML176 to 136R. Of the 200 BC1F1 (l36R/CML176//136R) plants genotyped, 104 were found to be heterozygous, producing products corresponding to both alleles (o2 and O2), while 96 produced a single band corresponding to the homozygous dominant (O2O2) condition. The ratio of the two groups in the backcross (BC) population was consistent with the proportion 1:1 accorded by the Chi-square test (X2=0.16< X2 0.05=3.84)'for a single gene in a BC population. Therefore, under the conditions of PCR used, SSR markers phi057 and phi 112 will constitute the framework for marker assisted introgression of the o2 trait into suitable maize genotypes in Uganda.
Key Words: QPM, tryptophan, Uganda, Zea mays