| Sharp eyespot of wheat(Triticum aestivum L.),which caused by the soil-borne fungus Rhizoctonia cerealis or Rhizoctonia solani,is present throughout the temperate wheat growing regions of the world.Typical symptoms of the disease are oval dark-bordered lesions on the leaf sheath and stem of young and mature plants.Severe infection of shoots may cause plant lodging or premature ripening(whiteheads),and affect quantity and quality of grain.Sharp eyespot has become a devastating disease in the Middle and Low Yangtze River Region and Huang-huai Winter Wheat Region of China,and is expanding to the northern area.Wheat resistance to sharp eyespot can be a potential means to reduce the needs for application of fungicides and agricultural inputs.Mapping quantitative trait loci(QTL)associated with sharp eyespot resistance can provide theoretical guide for wheat breeding.In this study,we collected ninety-five wheat accessions with different resistance to sharp eyespot for research.The phenotypic identification data was obtained by the trials which were conducted during the 2009-10,2010-11,2011-12,2013-14 and 2014-2015.The 95 wheat cultivars were classified into three genetic relative groups estimated by population structure analysis,which was consistent with their geographic eco-type.UPGMA cluster analysis revealed the presence of two major groups.The first group contained landraces,and the second group contained domestic improved varieties and alien varieties.Further analysis showed that most domestic improved varieties could be clustered into a subgroup when the genetic distance was 0.286.The cluster analysis displayed a similar clustering pattern of relationship to population structure analysis.We used 150 SSR markers to estimate the linkage disequilibrium(LD)of wheat in the whole genome level,and found that whether non-linear or intra-chromosomal marker pairs exists LD.LD between intra-chromosomal marker pairs was higher than the non-linear marker pairs.The LD decay distance ranged from 0 to 10 cent Morgan(cM),however LD was also existed when the genetic distance was greater than 10 cM.Marker-trait association analysis was calculated by mixed linear model(MLM)in software TASSEL 4.0.There were 19 SSR markers associated with sharp eyespot resistance under MLM program.Seven SSR loci on chromosome 1D,2B,2D,5A and 7D were identified more than twice,and the rate of explanation on the phenotype was 4.32?24.56%.Four marker loci were linked closely with the QTL located by previous.The QTLs which are respectively linked with markers Xwmc732-1D Xgwm415-5A and Xcfa2250-5A may be novel.According to Breseghello proposed "null alleles" method,the phenotypic allele effect was estimated through comparing the average phenotypic value over accessions with that of"null allele".Sixteen elite alleles were screened out,including the Xwmc732-300 with largest positive phenotypic effect and Xgdm67-121 with largest negative phenotypic effect.In order to discover elite wheat germplasm resource with stable resistance to sharp eyespot,88 resistant wheat accessions previously identified were re-evaluated in a three-year trial by artificial inoculation under field conditions.Thirty-two accessions among the tested showed resistant or moderately resistance to sharp eyespot,including 8 domestic improved varieties,6 landraces and 18 alien varieties.The genetic diversity analysis showed that alien varieties had highly diversity value than domestic improved varieties and landraces;Cluster analysis and principal coordinate analysis(PCA)indicated that landraces are distant from alien varieties and domestic improved varieties;The 32 resistant varieties were genotyped with 14 microsatellite markers,which were closely linked with identified QTLs for sharp eyespot resistance.Fewer of the known resistance QTL was detected in some cultivars,such as Wunong 148,Compare,Coker983,H-Line,Mason,Compare and Tyalt.They could be the novel resources to improve sharp eyespot resistance in wheat breeding. |
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