Genetic Analysis And QTL Mapping Of Cold Tolerance In Melon

Melon（Cucumis melo L.）is an important fruit of Cucurbitaceae.Because of its sweet taste and abundant nutrition,it is widely welcomed by the public.The demand for annual supply of melon is increasingly urgent.Although horticultural facilities have been popularized in Muskmelon production,early spring cultivation in northern China is still often encroached by low temperature.The "late spring cold" in the Yangtze River Basin has become an important environmental obstacle to muskmelon production in spring,and low temperature has become one of the important environmental factors restricting muskmelon cultivation in spring and annual supply in China.Breeding and popularizing melon varieties with strong cold tolerance is an effective way to solve this problem,but the premise is to clarify the genetic law of cold tolerance and explore the major gene QTL controlling cold tolerance.In this study,a strong cold-tolerant germplasm ZTG00581 was screened from the core germplasm of melon.The inheritance of cold-tolerance of the germplasm was analyzed,and the cold-tolerant QTL was mapped.The main results were as follows:In this experiment,low-temperature resistant material ZTG00581（581）and low-temperature sensitive material ZTG00906（906）were used as research objects to construct six generations（P1,P2,F1,B1,B2,F2）population and conduct multi-generational joint analysis;140 F2 plants and parents were sequenced by RAD-seq technology to construct genetic linkage map.The experimental results are as follows:1.Differential analysis of cold-tolerant and cold-sensitive parents.The chilling injury index of cold-tolerant parent ZTG00581（581）and cold-sensitive parent ZTG00906（906）were 0.39 and 0.86,respectively.Under low temperature stress,chlorophyll chlorophyll fluorescence parameters of 581 were generally higher than 906,Fo,Fm and QY_max were 1.48,1.82 and 1.94 times higher than 906,respectively;conductivity of 581 was significantly lower than 906,and its relative chlorophyll content SPAD and soluble protein content were higher than 906;ultrastructure observation The results showed that the damage degree of organelles such as chloroplast 906 was more serious than 581 under low temperature stress.2.Genetic analysis of cold tolerance.The cold tolerance of 5 81 x 906 combinations(distinguishing orthogonal（O）and backcross（R）seedlings in six generations（Pi,P2,Fi,B i,B2,F2）was analyzed by multi-generation combined genetic analysis.Under the two chilling injury indices of CⅡ₁ and CⅡ₂,it was found that the optimal genetic models of CⅡ₁O（Chilling injury index of the first statistics）and CⅡ₁R were two pairs of additive-dominant-epistatic major genes+additive-dominant polygenes,the optimum genetic model of CⅡ₂O（Chilling injury index of second statistics）is additive-dominant-epistatic model,and the optimum genetic model of CⅡ₂R is two additive-dominant-epistatic major genes+additive-dominant-epistatic polygene models.The results of both positive and negative crosses showed that cold tolerance of melon seedlings was a complex quantitative trait controlled by multiple genes.3.Construction of genetic map.A F2 population consisting of 140 individual plants was constructed and RAD-seq simplified gene sequencing was carried out for the population and parents.The parents 581 measured 5,788,680 high-quality reads,accounting for 93.57%of the total reads;906 measured 5,824,098 high-quality reads,accounting for 94.28%of the total reads;and the F2 population averaged 7,136,845 high-quality reads,accounting for 93.92%of the total reads.Compared with the reference genome of melon（DHL92）,379,585 SNPs were detected.;the screened SNP was further filtered and 6443 polymorphic markers were obtained;finally 566 Bin markers were screened out.By adding additional SSR markers for linkage analysis,a set of genetic maps including 12 linkage groups was constructed,covering the genome length of 3843.64 cM and the average distance between markers was 5.9 cM.The number of markers on different linkage groups ranged from 31 to 70,and the length ranged from 225.82 cM to 439.72 cM.4.Location of cold tolerance QTL.F2:3 family population was constructed based on 140 individual plants.Three independent cold tolerance tests were carried out for F3 seedlings.Mapping of QTL with CⅡ₁ and CⅡ₂,10 and 8 loci were detected,respectively,on chromosomes 2,3,4,6,9 and 11.The segments detected by both markers were between chr02 2920027 and chr02 2647384 markers on chromosome 2,and between CMSSR26454 and CMSSR26330 markers on chromosome 11.Among them,qCⅡ₁11-3 could explain 12.48%of phenotypic variation,and qCⅡ₁11-1 could explain 12.48%of phenotypic variation.The explanatory rate of phenotypic variation was 16.37%.