| dc.description.abstract | 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. | en |
