Formation of Heterotic Groups and Understanding Genetic Effects in a Provitamin A Biofortified Maize Breeding Program

Developing biofortified maize (Zea mays L.) cultivars is a viable approach to combating the widespread problem of vitamin A deficiency among people for whom maize is a staple food. To enhance CIMMYT's provitamin A maize breeding efforts, this study: (i) evaluated whether separation of experimental maize lines into groups based on maximizing their molecular-marker-based genetic distances (GD) resulted in heterosis for among-group crosses, (ii) assessed genetic effects (general and specific combining ability, GCA and SCA) for grain yield and provitamin A concentrations in hybrids among 21 inbred lines representing the three proposed groups, and (iii) assessed the association between grain yield and provitamin A concentrations. The lines were crossed following a partial diallel design resulting in 156 hybrids that were evaluated at four environments with two replications of one-row plots. The first plant in each plot was self-pollinated to produce grain for provitamin A analysis. Significant but small yield advantage of among-versus within-group crosses (0.47 Mg ha(-1), P < 0.05) suggested that the groups identified by maximizing GD could be a practical starting point for further breeding work to develop useful heterotic groups. Furthermore, the GD-proposed heterotic groups were improved by later revising some line assignments to groups using estimates of SCA effects. General combining ability effects were significant (P < 0.01) for all traits, whereas SCA effects were weak (P < 0.05) or not significant for provitamin A carotenoid concentrations, indicating that these were controlled primarily by additive gene action. Grain yield was not significantly correlated with provitamin A concentration, indicating that both traits could be improved simultaneously.