Characterization of the Genetic Diversity of Cercospora Zeina in Kenya and Mapping the QTL for resistance to Gray Leaf Spot and Turcicum Leaf Blight in Maize
OMONDI, Dennis Oduor
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Gray leaf spot (GLS) caused by Cercospora zeina in Africa and Turcicum leaf blight (TLB) caused by Exserohilum turcicum are a major threat to maize production due to the associated grain yield losses. Resistance in the currently grown maize hybrids could be overcome by high levels of genetic diversity that characterize C. zeina and E. turcicum populations. However, little is known concerning the population structure and diversity of C. zeina in Kenya. The objectives of this study were first to characterize the genetic diversity of C. zeina. Secondly to map the quantitative trait loci (QTL) conditioning resistance to GLS and TLB in the double haploid (DH) population from CML511×CML546. The genetic diversity of C. zeina and the role of sexual recombination in this population was determined by collecting GLS infected maize leaves from four counties in Kenya. The genomic DNA for the 129 successful isolates were assayed using previously designed mating type (MAT) primers and genotyped using 11 microsatellite markers. The CTB7 (cercosporin toxin biosynthesis 7) test confirmed that all the isolates sampled were C. zeina as they all produced PCR products of 618 bp. The population exhibited high levels of gene diversity (He=0.445), slightly high gene flow (Nm=3.85) and high level of polymorphism. In addition, the four counties were characterized by nearly equal distribution of the two mating types, providing evidence that it could be undergoing sexual recombination. Occurrence of sexual recombination could be responsible for the high genetic diversity. STRUCTURE analysis revealed that the population clustered into four sub-groups according to the four counties. The PhiPT value of 0.15 (p=0.001) corroborated with AMOVA tests was significant to provide evidence for partial population differentiation. QTL mapping was achieved by evaluating the DH population in Maseno and Kabianga in a 5×46 alpha lattice design during the long rains of 2018 and 2019. The disease incidence for the plots were scored on a scale of 1-9 and the best linear unbiased predictions determined using META-R statistical software. Marker genotyping of the population was performed using 1250 markers in diversity arrays technology (DArTseq). Linkage map construction and QTL analysis were conducted in QTL IciMapping v4.1. Nine GLS resistance QTLs were mapped on the chromosomal bins 1.06, 1.07, 1.11, 2.04, 2.06, 3.04, 3.05, 4.1 and 7.04. Fourteen TLB resistance QTLs were detected on the chromosomal bins 1.02, 1.08, 2.05, 2.06, 2.07, 3.01, 3.04, 4.02, 4.08, 5.03, 6.05, 7.03, 8.08 and 10.04. The QTLs were detected in at least two environments. The highest phenotypic variance was conditioned by qGLS1_190 (16.60%) for GLS and qTLB8_171 for TLB (13.65%). Disease resistance was negatively correlated with flowering time suggesting higher resistance in the late maturing genotypes. These findings will enhance proper identification of the pathogen causing GLS and GLS management programs. The identified QTLs and their flanking markers could be validated and fine mapped in future work for use in breeding for resistance to GLS and TLB.