The two-level marker assisted selection in BC2 generation of the conversion of standard maize lines to their QPM version

Abstract

Quality Protein Maize (QPM) is nutritionally and agronomically improved maize. In order to shorten the period required for development of QPM hybrids through the conventional method of backcrossing, marker assisted selection (MAS) is being used. After successful conversion of one commercial maize inbred line to its QPM counterpart for growing in temperate climate, this breeding program was continued at Maize Research Institute (MRI) with larger number of maize lines. Four commercial MRI inbreds, chosen for marker assisted introgression of the quality protein trait, and their BC2 progenies were subjected to two-level selection procedure. First, BC2 plants were analyzed with opaque2 (o2) specific molecular markers to identify heterozygotes. Second, the selected heterozygotes were screened with SSR markers distributed throughout the genome to identify genotypes with the highest recovery of recurrent parent’s genome (RPG). The specific markers identified 100 out of 192 plants (52%) as heterozygous. Genetic similarity values between parental lines and their BC2 heterozygous progenies were in the range from 0.77 to 0.99 (77-99% RPG). The highest proportion of RPG was found in L1 (93-99%) and the lowest in L3 progenies (77-89%). Average values for the RPG content ranged from 83.9 to 95.8%. Progenies with RPG above 95% were selfed to produce BC2F2 plants which will be subjected to foreground selection. This time selection will be focused on homozygous recessive individuals, given that the presence of opaque2 gene in the homozygous recessive state is the aim of the QPM selection. Finally, those o2o2 genotypes will be screened for biochemical and phenotypic traits to confirm their nutritional and agronomical superiority.