Genetic diversity and performance of maize varieties from Zimbabwe, Zambia and Malawi

Large scale and planned introduction of maize (Zea mays) in southern Africa was accomplished during the last 100 years. Since then, smallholder farmers and breeders have been selecting varieties best adapted to their specific growing conditions. Six studies were conducted to generate information on the current levels of genetic diversity and agronomic performance of both farmer-developed and commercially-bred maize varieties in Zimbabwe, Zambia and Malawi to help in the identification of sources of new alleles for improving yield, especially under the main abiotic stresses that prevail in the region. In the first study, 267 maize landraces were collected from smallholder farmers in different agro-ecological zones of the three countries for conservation and further studies. Passport data and information on why smallholder farmers continue to grow landraces despite the advent of modern varieties were also collected along with the landraces. The second study revealed considerable variation for phenological, morphological and agronomic characters, and inter-relationships among the landraces and their commercial counterparts. A core sample representing most of the diversity in the whole collection of landraces was selected for further detailed analyses. The third study revealed high levels of molecular diversity between landraces originating from different growing environments and between landraces and commercially-bred varieties. The Simple Sequence Repeat (SSR) data also showed that the genetic diversity introduced from the original gene pool from the USA about 100 years ago is still found in both the descendant landraces and commercially-bred varieties. The fourth study showed that in general, commercially-bred varieties outyielded landraces under both abiotic stress and nonstress conditions with some notable exceptions. Landraces were more stable across environments than improved varieties. The most promising landraces for pre-breeding and further investigation were also identified. The clustering patterns formed based on agronomic data were different from SSR markers, but in general the genotype groupings were consistent across the two methods of measuring diversity. In the fifth study, the more recently-bred maize varieties in Zimbabwe showed consistent improvement over older cultivars in grain yield. The apparent yearly rate of yield increase due to genetic improvement was positive under optimum growing conditions, low soil nitrogen levels and drought stress. The sixth study revealed that in general, genetic diversity in Zimbabwean maize has neither significantly decreased nor increased over time, and that the temporal changes observed in this study were more qualitative than quantitative. The results from the six studies confirm the origin of maize in southern Africa and reveals that considerable genetic variation exists in the region which could be used to broaden the sources of diversity for maize improvement under the current agro-ecological conditions in southern Africa