Maize is among the most commonly bred crops in the world and maize breeding programmes are
increasingly using molecular tools to enhance the efficiency and speed of developing productive
cultivars. Breeding efforts of CIMMYT have focused on incorporating drought tolerance into
elite germplasm. The incorporation of the Fbr1 gene into its elite germplasm was one such effort
as the improvement of drought tolerance relies on manipulation of adaptive traits that limit yield
under the target stress. The aim of this study was to find the genetic basis and effect of the Fbr1
tassel mutation on maize grain yield under stress and non-stress environments.
A number of elite CIMMYT lines have been successfully converted to the Fbr1 mutation, and
were homozygous for the 1074 SNP loci used, thus could be used in breeding programmes
involving these new tassel mutants. The UPGMA cluster analysis revealed two discrete clusters
of the inbred lines according to predefined CIMMYT heterotic groups. In the principal
component analysis, the SNP loci were effective in characterising the maize inbred lines since
they separated maize lines according to tassel size. Positive relationships between grain yield and
pollen yield and its components were found, under drought conditions. However, the Fbr1 tassel
trait did not have a positive effect on yield under stress and Fbr1 x Fbr1 hybrids had lower grain
and pollen yield, and were less adapted to abiotic stress conditions. This raised questions on the
value of incorporating such trait in breeding programmes targeting stress tolerance. Therefore,
incorporation of the Fbr1 tassel trait should accompany selection for other traits associated with
stress tolerance under low N and drought conditions, such as the âstay greenâ trait, factors
associated with premature senescence, synchrony between male and female flowering and
decreased barrenness if yield gain is to be realized. Estimates of genetic components of variance
revealed importance of both additive and dominance components in the determination of grain
yield, pollen yield and their components. Additive gene action was predominant in determining
tassel size and pollen yield, thus progress can be made by selecting within segregating progenies
when improving maize populations for the Fbr1 trait.
The investigation of GxE interaction showed significant variation in stability of Fbr1 lines and
hybrids as measured by mean yield and ASV. The AMMI and the GGE biplots ranked the best
and poorest genotypes similarly in terms of yield performance and stability. The two
classification models could be used simultaneously to make selection of genotypes more precise. Genetic distances were positively correlated with hybrid performance, SCA, and heterosis
indicating that genetic distance could accurately predict hybrid performance in this set of
germplasm.
This study showed that yield is a complex trait and its improvement under stress should involve
simultaneous selection of other traits associated with stress tolerance. The SNP markers are the
marker of choice in genetic characterisation and determination of marker-based genetic distances
because of their wide coverage of the maize genome. A number of lines homozygous for the
Fbr1 tassel trait, has been identified in this study. These lines could be used in future research
such as the developing of mapping populations aimed at tagging the Fbr1 trait, since the
position of the Fbr1 gene in the maize genome is still unknown.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-07232013-101911 |
| Date | 23 July 2013 |
| Creators | Dari, Shorai |
| Contributors | Dr A Minnaar-Ontong, Dr J Macrobert, Prof MT Labuschagne |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-07232013-101911/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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