Genetic and Virulence Variability of Zymoseptoria Tritici Populations and Genome Wide Association Studies for Septoria Tritici Bloch Resistance and Yield Potential in Wheat (Triticum Aestivum L.) in Ethiopia
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Date
2021-08-25
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Addis Ababa University
Abstract
Septoria tritici blotch (STB) caused by the fungus Zymoseptoria titici is the major bottleneck to wheat production in Ethiopia and worldwide. The present study was targated to investigate genetic and virulence variability of Zymoseptoria tritici populations; and Genome-Wide association studies for STB resistance and yield potential in bread wheat in Ethiopia. Genetic structure analysis of 182 Z. tritici isolates representing eight populations from major wheat growing areas of Ethiopia using 14 polymorphic microsatellite markers confirmed the presence of high genetic diversity (h = 0.45) where 92% of the total genetic variation resides within populations. Cluster (UPGMA), PCoA and STRUCTURE analyses did not group the Z. tritici populations into sharply genetically distinct clusters according to their geographical areas of sampling. All individual samples shared alleles from two subgroups (K=2). Pathogenicity assay conducted with eight Z. tritici isolates on a set of 45 bread wheat genotypes showed highly significant differences for isolate-by-genotype interaction on both percentage of necrosis leaf area (NLA) and pycnidia coverage (PC). The isolates showed significantly higher differences in virulence patterns. Isolate I1 sourced from Bale was found to be the most virulent (93%) and aggressive (NLA = 58%) isolate. Among tested Stb genes, Stb16 conferred broad-spectrum resistance to 75% of the isolates. Three genotypes (MURGA, Km7 and Hidase) conferred the greatest level of seedling resistance to the tested isolates. Molecular screening for major Stb genes in 180 genotypes using 16 tightly linked markers identified a number of Stb genes ranging from 6.67% for Stb12 to 96.1% for Stb13 gene. Field evaluation conducted on 180 wheat genotypes across six environments revealed a very highly significant difference among the wheat genotypes for most traits. Top 5% selected genotypes showed 10.8 - 21.99% and 52.25- 74.25% superior grain yield performance and STB resistance to standard check (King-bird), respectively. Moreover, population structure, linkage disequilibrium (LD), and GWAS for adult-plant STB resistance, yield, yield- related and phenological traits were conducted for 178 wheat genotypes using 7,776 SNPs resulted from Illumina HiSeq2500 Genotype-by-sequencing.
STRUCTURE and PCA confirmed two sub-populations with greater degree of genetic admixture. In all chromosomes, LD between pairs of SNPs declined at r2 = 0.2 within physical distance of 2.26 -105.62 Mbp, with an overall mean of 31.44 Mbp.The association analysis identified 53 loci significantly associated with STB resistance pointing to 33 putative QTLs. Most of these shared similar chromosome locations with already published Septoria resistance genes and QTLs, and were distributed across chromosomes 1B, 1D, 2A, 2B, 2D, 3A, 3B, 3D, 4A, 5A, 5B, 6A, 7A, 7B and 7D. However, five of the putative QTLs identified on chromosomes 1A, 5D and 6B appeared to be novel. Predicting the possible functions of the detected loci from the IWGSC RefSeq Annotation v2.1 revealed the existence of disease resistance-associated genes in the identified QTL regions which are involved in plant defense responses. Seven of the QTLs for STB resistance also co-localized with MTAs for agronomic traits. In conclusion, the study provides new insights into the genetic structure and virulence variability of Z. tritici in Ethiopia to design effective and durable management strategies. The identified Stb genes, QTLs and resistance source materials could be used in wheat genome-assisted resistance breeding against STB. Therefore, the information generated in the present study is useful to improve STB resistance and grain yield performance in wheat which ultimately leads to increased and stable wheat production to ensure food security in Ethiopia and the region at large.
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Keywords
Dartseq, Genetic Diversity, Gwas, Ld, Pca, Qtl, Ssr Marker, Stb Genes, Stb, Virulence Variability, Wheat, Zymoseptoria Titici