Two-generation marker-aided backcrossing for rapid conversion of normal maize lines to quality protein maize (QPM)

The low nutritive value of maize endosperm protein is genetically corrected in quality protein maize (QPM), which contains the opaque2 gene along with numerous modifiers for kernel hardness. We report here a two generation marker-based backcross breeding program for incorporation of the opaque2 gene along with phenotypic selection for kernel modification in the background of an early maturing normal maize inbred line, V25. Using the flanking marker distances from opaque2 gene in the cross V25xCML176, optimum population size for the BC2 generation was computed in such a way that at least one double recombinant could be obtained. Whole genome background selection in the BC2 generation identified three plants with 93 to 96% recurrent parent genome content. The three BC2F2 families derived from marker identified BC2 individuals were subjected to foreground selection and phenotypic selection for kernel modification. The tryptophan concentration in endosperm protein was significantly enhanced in all the three classes of kernel modification viz., less than 25%, 25-50% and more than 50% opaqueness. BC2F3 lines developed from the hard endosperm kernels were evaluated for desirable agronomic and biochemical traits in replicated trials and the best line was chosen to represent the QPM version of V25, with tryptophan concentration of 0.85% in protein. The integrated breeding strategy reported here can be applied to reduce genetic drag as well as the time involved in a conventional line conversion program, and would prove valuable in rapid development of specialty corn germplasm.