Genetic Analysis And Gene Mapping Of Resistances To Downy Mildew,Powdery Mildew And Angular Leaf Spot In A Cucumis Hystrix Introgression Line Of Cucumber

Cucumber(Cucumis sativus L.)is an economically significant vegetable crop that cultivated worldwide.Downy mildew,powdery mildew and angular leaf spot are three important leaf diseases in cucumber,which posed huge threat on production of cucumber.Deployment of cucumber resistant cultivars with resistances to downy mildew,powdery mildew and angular leaf spot is the most economic and environment-friendly method for controlling the three diseases in cucumber breeding.However,most of the cucumber resistant materials only have single resistance or double resistances to downy mildew,powdery mildew and angular leaf spot.Cucumber germplasm resources with three kinds of resistances to downy mildew,powdery mildew and angular leaf spot were rarely reported.In this study,based on the evaluation of diseases resistance in different locations for several years,a Cucumis hystrix introgression line of cucumber IL52 with resistances to downy mildew,powdery mildew,angular leaf spot was identified.The F1,193 F2 plants and 155 recombinant inbred lines(RILs)were constructed by cucumber inbred line CCMC with susceptibility to downy mildew,powdery mildew,angular leaf spot and IL52.Genetic analyses of downy mildew,powdery mildew and angular leaf spot resistance in IL52 were respectively performed based on the phenotypic data of natural epidemics on the plants at the adult plant stage in the field.Gene mapping of downy mildew,powdery mildew and angular leaf spot resistances in IL52 were respectively conducted by combining BSA-seq analysis and traditional QTL/gene mapping method.The genetic relationship of double disease resistances to downy and powdery mildew in IL52 was also illustrated.The candidate genes for resistance to downy and powdery mildew were predicted.The closely linked markers with the angular leaf spot resistance gene in IL52 were also identified.The main contents and results were shown as below:1.Genetic analysis and QTL mapping of downy mildew resistance in IL52Based on the statistical results of phenotypic data of natural epidemics of downy mildew exposed on the CCMC,IL52 and 155 RILs at adult plant stage in the field for 3 years,genetic analysis indicated that the downy mildew resistance in IL52 was controlled by polygenes.Following the method of BSA-seq analysis,4 QTLs for downy mildew resistance were detected,which were DM5.1,DM5.2,DM5.3 and DM5.4,respectively.Of them,DM5.3 was a reliable major-effect QTL for downy mildew resistance with an average?(SNP-index)=1.Traditional QTL mapping analysis identified 6 QTLs including dm1.1,dm1.2,dm1.3,dm5.1,dm5.2 and dm6.1 for downy mildew resistance.Among them,dm5.1 and dm5.2 were major-effect QTLs,the other QTLs were minor-effect.The genomic regions of DM1.1,DM5.1,DM5.3 identified by BSA-seq analysis were consistent with the chromosome intervals of dm1.2,dm5.1,dm.5.2 detected by traditional QTL mapping method respectively,which suggested that the QTL mapping analysis for downy mildew resistance in this study was reliable.Furthermore,as compared with the results from Pang et al.(2013),the genomic region of downy mildew resistance major-effect QTL dm5.2 in the present study was validated and narrowed down.dm5.2 was located within 22.66-25.75 Mb on chromosome 5,which was linked with marker InDel73.2.Genetic analysis and gene mapping of powdery mildew resistance in IL52Based on the statistical results of phenotypic data of natural epidemics of powdery mildew exposed on the CCMC,IL52,Fi,193 F2 plants and 155 RILs at adult plant stage in the field,genetic analysis indicated that the powdery mildew resistance in IL52 was controlled by a single recessive gene pm.Using the 155 RILs,pm was initially mapped into 23.21-25.70 Mb on chromosome 5 by combining BSA-seq analysis and traditional gene mapping method.Fine mapping of pm was further narrowed into 24,550,703-25,018,946 bp by using 193 F2 plants and newly developed polymorphic markers based on BSA-seq analysis.A perfectly co-segregating marker SNP6 with pm was also identified.SNP6 is present in the 3'UTR of Csa5M622830.1,which may be a likely candidate gene for powdery mildew resistance in IL52.3.The study of genetic relationship of the double disease resistances to downy and powdery mildew in IL52dm5.2 and pm were co-localized in the same physical interval on chromosome 5,which indicated that dm5.2 and pm were linked.The co-localized locus dm5.2/pm conferred partial resistance to downy mildew and complete resistance to powdery mildew.Based on BSA-seq analysis,the three candidate genes(Csa5M622800.1,Csa5M622830.1,Csa5M623490.1)for powdery mildew resistance were the same with three of seven candidate genes for downy mildew resistance,even the same as the amino acid change.The three genes were predicted as the candidate genes for resistance to downy mildew or powdery mildew.4.Genetic analysis and gene mapping of angular leaf spot resistance in IL52Based on the statistical results of phenotypic data of natural epidemics of angular leaf spot exposed on the CCMC,IL52 and 155 RILs at adult plant stage in the field,genetic analysis indicated that the angular leaf spot resistance in IL52 was controlled by a single recessive gene psl which was mapped into 3,118,855-3,951,558 bp on chromosome 1 by combining BSA-seq analysis and traditional gene mapping method.The polymorphic marker ALS-InDel developed from BSA-seq analysis was closely linked to psl by genetic linkage analysis in the 18 resistant RILs,18 susceptible RILs and 19 susceptible cucumber inbred lines,which will be useful for marker-assisted breeding of angular leaf spot resistance in cucumber.