Genetic Diversity among African Lowland Maize Accessions Assessed by Morphological Traits and Simple Sequence Repeat (SSR) Markers

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Information on genetic diversity among landraces can lead to identification of new alleles for maize improvement. Owing to growing concerns of climate change, reduced arable lands, population increase and increased maize usage in Africa, there is the need to explore our landraces which are believed to harbour rich reserves of alleles for crop improvement towards ensuring food security. The study sought to identify variability and estimate the genetic diversity in the African lowland maize population by means of agrophenomorphological and SSR markers. Sixty-four accessions originating from eight countries in Africa, spanning latitude 8.85o S in Tanzania to 12.9o N in Chad and longitude 39.3oE to 10.7oW at elevation range of 50 to 700 m.a.s.l. were evaluated for quantitative and qualitative traits. Forty-seven accessions evaluated on 31 traits constituted morphological study. The analysis of variance revealed significant differences (P<0.001) among accessions for all measured traits except cob colour, kernel thickness and ear number. The predominant qualitative traits of the accessions were pale yellow silk colour, regular kernel arrangement, mixed grain colours and flint grains borne on white cobs. Among the quantitative traits, the most variable quantitative trait was plant height and the least was ear position. With regards to earliness days to 50% anthesis ranged from 44 to 61 days and days to 50% silking from 50 days to 69 days, Anthesis-silking interval ranged from 3 days to 8 days. Grain yield varied from 2.1 to 6.3 Mgha-1 with a mean of 4.0 Mgha-1. Accessions which combined earliness of 47-50 days and high yield of 3.8-5.87 Mgha-1 were TZm-1505, TZm-49, TZm-295, TZm-1427, TZm-1503, TZm-386, TZm-343 and TZm-1522. The population was characterised by low to moderate broad-sense heritability of traits from 5% in kernel thickness to 44% in plant height. Earliness and grain yield exhibited low heritability estimates of 16% and 27% vi respectively while a moderate heritability of 35% was revealed in anthesis-silking interval. Strong positive genetic correlation between grain yield and ear leaf width of 0.66 and ear leaf length of 0.57 were remarkable indicating correlated response to selection for grain yield. The genetic distance ranged from 0.00 to 0.88 with a mean of 0.28±0.19. Cluster analysis grouped the genotypes into three clusters. Grain yield in cluster I was controlled by the large values of ear characteristics whereas in cluster II, grain yield was driven by ear leaf length and ear leaf width. Cluster III was good for earliness. The first three PCA explained 79.10% of the total variance and the major contributors were plant height, ear leaf length, ear leaf width and grain yield. Using SSR profiling, 64 accessions were evaluated across the 10 chromosomes of maize using sixteen primer sets. A total of 2,216 alleles ranging from 114 to 228 were detected across the genotypes with a mean of 170.46. Across the loci, number of alleles ranged from 2 to 10 with an average of 5.46. Observed heterozygosity ranged from 0.00 to 0.86 with a mean of 0.46 ± 0.30 while expected heterozygosity ranged from 0.26 to 0.78 with a mean of 0.65±0.14. The analysis revealed that on the basis of chi-square goodness-of-fit test, the observed heterozygosities and the expected heterozygosities were not significantly different. Pair-wise genetic dissimilarity coefficient ranged from 0.30 to 1.00 with an average of 0.70±1.0. A UPGMA clustering produced two main clusters irrespective of the place of origin. The unique accessions identified would be useful in maize improvement with regards to earliness, drought tolerance and grain yield. The higher heterozygosity and alleles identified confirmed that the African landraces are a rich source of unique alleles yet untapped.
A thesis submitted to the Department of Biochemistry and Biotechnology, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements Master of Philosophy degree in Biotechnology.