| Developing biofortified maize cultivars is a promising approach to overcome the widespread problem of vitamin A deficiency in the developing world. The objectives of a first study were to: (1) evaluate whether molecular marker-based genetic distance separation of maize lines into heterotic groups results in heterosis among groups that could further be developed into a useful heterotic pattern, and (2) assess gene action (general and specific combining ability, GCA and SCA) for grain yield and provitamin A concentrations among inbred lines representing putative heterotic groups. A second, association mapping study was conducted to (3) identify genes and genic regions controlling variation for carotenoid concentrations, (4) use additive linear models of selected SNP markers to predict carotenoid concentrations of lines in breeding programs, and (5) assess the suitability of association mapping analysis models using four association mapping panels. To address objectives (1) and (2), 21 lines were crossed following a modified North Carolina Design II with six sets, where sets 1-3 contained crosses between putative heterotic groups, and sets 4-6 were crosses within groups. The resultant 152 hybrids were evaluated in two-replicate trials at four environments in Mexico. Significant but small yield advantage of among versus within putative heterotic group crosses (0.4 Mg ha-1, P<0.05) confirmed that genetic distance can be useful, but that further breeding work would be needed to develop these groups into a useful heterotic pattern. GCA effects were significant for both provitamin A concentration and grain yield, whereas SCA effects were significant only for grain yield, indicating that provitamin A concentration is controlled primarily by additive gene action. For objectives (3) and (4), association mapping identified the zeaxanthin epoxidase gene (R2=0.14), and a significant marker (R2=0.10) located close to the β-carotene hydroxylase gene, CrtRB1, as important regions determining carotenoid phenotypes. Additive linear models using selected SNPs accurately predicted carotenoid concentrations of maize lines (r≥0.8, P<0.01). For objective (5), the association mapping panels identified the phytoene synthase and the opaque-2 genes. Results of these field and molecular studies provided useful insights to enhance the effectiveness of provitamin A breeding efforts in maize. |
