QTL Analysis Of Anthracnose (Colletotrichum Acutatum) Resistance In Capsicum

Anthracnose caused by Colletotrichum spp. is one of the most serious diseases in Capsicumproduction, mainly causing pre-and post-harvest fruit rot, leading to heavy yield and income losses.Breeding varieties with better disease resistance is the most economical and effective method forcontrolling various diseases, and also is one of the main breeding targets to improve the Capsicumdisease-resistance. We have constructed an interspecific linkage map from a BC1population of C.annuum×C. chinense segregating in anthracnose resistance in this study and identified QTLsassociated with anthracnose resistance at mature green and red stages. Results of this study wouldfacilitate molecular marker assisted selection (MAS) in Capsicum anthracnose resistance breeding.The main contents and results in our study are as follows：(1) Anthracnose resistance assay and genetic analysis. By morphological observation andrDNA-ITS sequencing, the Colletotrichum fungus we isolated was identified as C. acutatum，whichis more virulence than other Colletotrichum fungi. The BC1mapping population was derived from aninterspecific cross of inbred line77013(C. annuum)(recurrent parent susceptible to C. acutatum) andPBC932(C. chinense)(male parent with strong resistance to C. acutatum). The anthracnoseresistance evaluation was performed on detached mature green and red fruits with C. acutatum, usingthe microinjection method. Results of the evaluation of parents and F1indicate that PBC932wasstrong resistant to C. acutatum and resistance at both stage was inherited dominantly. The segregationratio of resistance and susceptibility was44:130,27:101at mature green and red fruit stage,respectively, both of which significantly fit a1:3Mendelian model, indicating that the resistance ofPBC932to C. acutatum may controlled by two complementary dominant genes at both stages. Thesegregation ratio RR: RS: SR: SS in BC1population was10:14:17:84, which not fit the1:3:3:9ratios,suggesting that the resistance genes in mature green and red stage were not independent. Meanwhile,the continuous distributions of all6indexes to resistance in BC1population suggest that minor genescould exist and affect the resistance.(2) Polymorphic marker selection and linkage map construction.186plants were selectedrandomly for linkage map construction. Out of815SSR,228CAPS and1InDel,365SSR,40CAPSand1InDel performed polymorphic between parents, showing a polymorphism rate of38.89%.Under a LOD threshold of3.0, an interspecific linkage map composed of14linkage groups wereconstructed, including350SSR,35CAPS and1InDel (marker linked with anthracnose resistance atmature green stage). This map covers a length of1310.2cM, and the number of markers in eachlinkage group varying from4to68, while length varying from21.8cM to137.4cM. The averagedistance between two markers was3.40cM in this linkage map. When compared with Capsicumlinkage group published,13linkage groups were assigned to11Capsicum chromosomes.Chromosome P8was missing in this map, and present as pseudolinkage linkage with markers on P1.(3) QTLs analysis of anthracnose resistance. Under a LOD threshold of2.0, inclusive compositeinterval mapping undertaken on genotyping and phenotyping data revealed9QTLs and3QTLs for resistance to C. acutatum at mature green and red fruit stage, respectively, locating at chromosome P3,P5, P7, P10and P12. Three main effect QTLs AnRGO5, AnRGT5and AnRGD5for overall lesiondiameter (GO), true lesion diameter (GT) and disease incidence (GD), respectively, were all mappedin the same marker interval of InDel and HpmsE116on chromosome P5, explaining62.38%,60.50%and33.17%of phenotypic variance in GO, GT and GD. Two main effect QTLs AnRRO5and AnRRT5for overall lesion diameter (RO) and true lesion diameter (RT) respectively, were also mapped in thesame marker interval of InDel and HpmsE116on chromosome P5, explaining15.24%and15.90%ofphenotypic variance in RO and RT. The resistance allels of all these five main effect QTLs comefrom resistance parent PBC932and contribute to the increase of resistance to C. acutatum. The samelocated of main effect QTLs, nearest to marker InDel, for mature green and red stage, indicated thatresistance to C. acutatum at different stage may controlled by the same major gene or different butclosely linked major genes, and affected by minor genes.These results provide reference for Capsicum resistance breeding against anthracnose and MAS.The information of major QTLs on chromosome P5may facilitate fine-mapping and clone ofanthracnose resistance genes.