Evaluation of F₂ × F₂ and ВС, × BC, maize interpopulation crosses

Further improvement in the performance of elite maize (Zea mays L.) hybrids is an important objective of maize breeding programs. Introgression of broadbase germplasm to improve the elite single cross B73 X Mol7H was considered. Estimates of genetic parameters in a [B73 X В Composite]F₂ X [Mol7H X A Composite]F₂Design 2 population and a [B73(B73 X В Composite)]BC, X [Mol7H(Mol7H X A Composite)lBC, Design 2 population were obtained. Proportion of broadbase germplasm had a linear effect on means with the order of performance (most favorable to least favorable) for all traits being B73 X Mol7H, the BC, interpopulation cross, the F₂ interpopulation cross, and A Composite X В Composite. Estimates of half-sib covariances (CovHS) and specific combining ability variance (VarSCA) for grain moisture and plant and ear heights were greater in the F₂ than in the BC₁ interpopulation cross as expected for a one-locus, two-allele genetic model. Contrary to expectations, estimates for grain yield of CovHS in the population related to B73 and of VarSCA were two and four times greater, respectively, in the BC, than in the F₂. The large estimates of CovHS and VarSCA for grain yield in the BC, interpopulation cross were consistent with expectations for a model in which B73 and Mol7H are homozygous for an allele at a modifier locus, which causes complete dominance at a quantitative trait locus. Frequency of the modifier allele in the two composites would be near zero under this model. The VarSCA results suggested the use of backcross populations for selection procedures that exploit specific combining ability effects. Together with higher frequencies of ВС₁ X BC₁ test-crosses superior to B73 X Mol7H, they indicated a higher probability for immediate derivation of superior single crosses from the BC₁.