Genetic diversity studies of tropical Ipgri maize (Zea mays L.) Landraces held in iita by agromorphological and simple sequence repeats marker evaluation

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November, 2016
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Genetic diversity among maize (Zea mays L.) landraces reveals genetic backgrounds with respect to alleles, polymorphisms and heterozygosities, as well as relationships among genotypes. There has always been the need in Africa to identify useful alleles for maize improvement in a wide genetic base yet little is done to search for diversity among existing maize landraces. The IPGRI landraces of the IITA maize collection has neither record of geographical origin nor information on genetic diversity. The research objective was to estimate the level of genetic diversity, determine relationships among the landraces, and reveal evolutionary processes that have contributed to the genetic status of the population. A total of 60 landraces and a check, ‘Obatanpa GH’ were evaluated by agromorphological characterization on 5 qualitative and 24 quantitative traits. Except for cob colour which was least variable with 98.0 % white and 2.1 % red, a large variability was observed for silk and grain colour, kernel texture and kernel arrangement Kernel arrangement with fairly equal distribution of straight, regular, irregular and spiral types was the most variable. On quantitative evaluation, large variability was demonstrated for all traits except number of ears per plant. Earliness ranged from 39 to 74 days with a mean of 54.8 ± 6.2 days to 50 % anthesis while days to 50 % silking covered 44 to 78 days and mean of 57.6 ± 6.3 days. Six early-maturing genotypes identified were TZm-149, TZm-1148, TZm-1150, TZm-1157, TZm-1153, and TZm-1152. Mean anthesis-silking interval revealed genotypes for drought tolerance having 1.2 to 1.4 days of anthesis-silking interval in TZm-1188, TZm-1183, and TZm-1106. Many individual plants of these accessions exhibited protogyny. Mean grain yield ranged from 2.16 ± 0.4 Mgha-1 to 6.19±1.7 Mgha-1 of which the best performers with yield exceeding 4.2 Mgha-1 were TZm-1185, TZm-1142, TZm-1213, TZm-1129, TZm-1143, TZm-1215, TZm-1150, TZm-1211, TZm-1152, TZm-1101, TZm-1123, TZm-1100, TZm-1138, TZm-1112, TZm-1212, TZm-1130, TZm-1190, TZm-1118, TZm-1106, TZm-1144, TZm-1122, TZm-1125, TZm-1117, TZm-1119 and TZm-1139. Low to moderate broad sense heritability estimates of 0.00 for stay green and ear weight to 0.68 and 0.69 for earliness were recorded. The medium to high heritability estimates signify traits are under control of minimal additive and some dominance gene effects for a slow pace in progress in breeding. Besides the strong positive correlation of yield components with grain yield, all other correlation coefficients with grain yield were weak and nonsignificant (P≤0.05). Genetic similarities ranged from 0.00 to 0.80 with a mean of 0.14±0.15 indicating extensive genetic diversity. The UPGMA cluster analysis grouped genotypes into two main heterogeneous clusters, cluster I having early-maturing, short plants with high grain yield and low anthesis-silking intervals whereas cluster II was of tall plants with poor grain yield. The first two principal components explained 85.0 % of the total variance with large contributions from plant height, ear height, anthesis, silking, ear leaf length, grain weight, grain yield, ear position, hundred kernel weight, kernel length, and kernel width. SSR profiling of 64 IPGRI genotypes at 12 loci produced a rate of polymorphism of 85.7 %, a total of 1,826 alleles ranging from 108 to 216. The number of alleles per locus ranged 3 to 10 with mean of 5.64±2.15 indicating lots of variability. The mean observed heterozygosity of 0.36±0.18 was not significantly different from the expected heterozygosity of 0.69±0.08, an indication of substantial mutation rate and polymorphism maintained by balancing selection. The high heterozygosity is also suggestive of a historical admixture event. Genetic distance by means of DICE similarity coefficient was 0.49±0.14. UPGMA clustering grouped the accessions into six clusters from which hybridization could be exploited. The large variability, polymorphism, and heterozygosity identified by both agromorphological and molecular assessments affirm the existence of wide genetic diversity in the IPGRI genotypes and their possible beneficial contributions if exploited in maize improvement programmes.
A thesis to be submitted to The Department of Crop and Soil Science, Knust, Ghana in partial fulfillment of the requirement for the award of Master of Philosophy in Agronomy (Plant Breeding).