Genetic analysis of simple sequence repeat markers and morphological traits variation in lowland, mid-altitude and highland African maize (Zea mays L.) accessions.

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Genetic diversity is a valuable resource for improvement in crop productivity and trait performance. Maize (Zea mays L.) is an important food security crop in Africa constrained with low yield typically below world average. The forces of population escalation, diminishing arable land, and climate anomalies with their attendant low yield and crop loss arising from drought and emerging diseases indicate worsening food insecurity in Africa. However, genetic improvement in a wide genetic base is a relevant and logical strategy requiring availability of large reserve of alleles bearing these traits. This strategy necessitates studies into the well-adapted traditional African maize germplasm to identify, quantify, and explore the basis of variation in order to reveal the historical processes that have created and driven the level of variation for efficient exploitation. The main objective of this research was to investigate genetic diversity of maize originating from twelve countries, covering a wide geographical region in three mega environments in Africa, namely, lowland, mid-altitude and highland accessions. The accessions were tested under non-stressed environments in Ghana by evaluation of 29 agro-morphological traits on 35 genotypes, and 16 simple sequence repeat markers on 57 accessions held in IITA Genetic Resource Center. Accessions showed wide variations in silk color, kernel arrangement, principal grain color, and kernel texture except cob color. Significantly different mean squares and large coefficient of variation indicated substantial variation among the genotypes for all traits except anthesis-silking interval. Variability was highest in mid-altitude followed by lowland, and was least in the highland accessions. Being the most important traits, earliness and grain yield varied from 49 to 66 days to anthesis and 1.7 to 6.2 Mgha-1, respectively. Anthesis-silking interval varied from 2 to 6 days. The study identified a single early-maturing genotype TZm-1376, with strikingly short ASI of 2 days which also possessed high yield of 5.6 Mgha-1 in consonance to the improved check ‘’Obatanpa GH’’ with yield of 6.3 Mgha-1.. Unusual combination of early-maturing yet high-yielding accessions were identified in ten genotypes TZm-4, TZm-41, TZm-270, TZm-1521, TZm-275, TZm-14, TZm-33 TZm-37, TZm-1367, and TZm-1376. Medium - maturing but high yielding genotypes which can be incorporated into breeding programs for improvement included TZm-1434, TZm-1356, TZm-1358 and TZm-242. The broad sense heritability estimates were low for all traits in the accessions except earliness traits in the lowland genotypes. Significant (p≤ 0.01) positive genotypic correlations of grain yield with hundred kernel weight, kernel length, tassel length, and ear leaf width indicated that selection for these traits will lead to simultaneous increase in grain yield. Morphological genetic similarity measures ranged from 0.00 to 0.80 with overall mean of 0.26 indicating wide genetic variability among accessions. The African landraces were 23 %, 29 %, and 38 % similar in the mid-altitude, lowland and highland accessions which agreed with the level of variability revealed by the descriptive statistical measures. A total of 70 alleles ranging from 2-10 with a mean of 5.38 alleles per locus for the 14 SSR loci were identified. The total number of alleles generated across the loci was 1,908 over 57 accessions. Polymorphism Information Content ranged from 0.18 to 0.81 having an average of 0.64 and 93 % polymorphism rate. The high average expected heterozygosity of 0.64 indicates abundance of heterozygosity probably arising from historic admixture of two or more divergent populations. Molecular analysis revealed average dissimilarity coefficients of 0.70 for mid-altitude, 0.69 for lowland and 0.65 for highland accessions with an average of 0.70 ranging from 0.00 to 1.00. These were consistent with the low similarity values produced by the morphological analysis. The UPGMA produced four and three main clusters for agro-morphological and SSR analysis, respectively, confirmed by the principal components biplots. Potential good clusters for exploiting heterosis in maize breeding programs were identified. The study has revealed wide genetic diversity in the accessions to permit their utilization as sources of alleles for improvement in performance and productivity of maize in Africa.
A thesis submitted to the Department of Crop and Soil Sciences of the faculty of Agriculture, Kwame Nkrumah University of Science and Technology, in partial fulfilment of the requirement of the award of Master of Philosophy in Agronomy (Plant Breeding).