Quantitative trait loci analysis of maize (zea mays l.) for maize streak virus resistance
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Maize streak virus disease (MSVD) is widespread in Kenya but is more serious in midaltitude areas and mid highland zones where epidemics have been experienced contributing up to 100% yield loss in maize. MSV is transmitted by virus of the family Germiniviridae in sporadic epidemics. The disease may cause up to 100% yield loss in susceptible crops. Various genetic factors for the resistance to the virus have been identified. The development of maize germplasm that is resistant to MSV has been thought as the main intervention in various breeding and crop improvement programs. Currently 8 QTL have been detected on chromosome 1,2,3,5 and 2 QTL on chromosome 6, 8 and 10. The objective of this study was to screen the F2 mapping population for resistance to MSVD by artificial inoculation using viruliferous C. mbila and mapping additional quantitative trait loci (QTL) controlling the MSV resistance. QTL analysis for MSV resistance was done using the F2 segregating populations derived from the single cross of two inbred lines VL073311 xCLRCW92. A total of 100 F2 lines with the two parental lines were planted at Kenya Agricultural and Livestock Research Organisation (Muguga South). Experimental design of an incomplete block of 108 entries having 107 entries and 1dummy entry was developed. The block consisted of 12 entries each of 9 blocks with 2 replicates. Inoculation was done at three leaf stage, MSVD disease severity was determined 21 days after inoculation. Disease level was scored using scale of 1-5 (where 1 was for the plants without disease symptoms and score 5 for the plants with complete destruction).74 progeny lines were found to be MSV disease resistant with resistance scores less or equal to 2.0 under artificial infestation in the field. The phenotypic data collected from F2 mapping population was analyzed with SAS software using analysis of variance (ANOVA) to determine the significance of the disease score (MSVD) in the two replicates. Analysis of variance showed that the disease severity scored on the scale of 1-5 was significantly different (P:::;0.000 I) among the maize lines. Maize line 32 had the highest disease severity of 3.5 which was significantly different from the rest while maize line 64 and maize line11 had the lowest severity of 1.5 which was significantly different from the rest. Single Nucleotide Polymorphisms (SNPs) markers were used based on the KASpar genotyping assay of KBiosciences. The linkage map was constructed using joinmap ver4.1 and the ratios of the segregated population were tested by Chi- square goodness- of- fittest. The final linkage map comprised of 100 individuals and 61 SNP markers distributed in ten linkage maps and 'covered a distance of 437.282cM. One QTL located in linkage group 4 was detected with a LOD score of above 2.0 with two SNP markers (PZA00413_20 and PZA03198_3) tightly linked to the QTLs. In a genome-wide association study, two SNPs were significantly associated with the maize streak virus on chromosome 3 explaining together 14 % of the phenotypic variance. The SNPs significantly associated with MSV resistance can be used in marker assisted selection and can accelerate the breeding process for the development of MSV resistant maize genotypes. These strategies will be recommended to programs that emphasize MSV resistance in maize hybrids.