| Melon is one of the world's top ten popular fruits,widely cultivated in China.As early as the "Shen Nong's Materia Medica" in the records of melon pedicle is a top grade herbs,its cultivation history has been more than a thousand years.Melon is thermophilic plant,sensitive to low temperature,during the whole growth period requires higher temperature.However,in the process of over winter and early spring cultivation,melon seedlings are often affected by low temperature,resulting in seedling extension,fruit quality decline,so the low temperature resistance of melon research is very important.The study on the cold-tolerance evaluation system of melon seedling and the whole genome-wide correlation analysis of cold-tolerance based on melon core germplasm were carried out.The results are as follows:(1)Establish a system for evaluation of chilling tolerance in melon seedlings:Nineteen melon germplasms with different genetic background were used as experimental materials and their seedlings were measured for 9 morphological indices and 8 physiological indices under low temperature stress.Multiple statistics analysis(principal component analysis,cluster analysis,and regression analysis.)were applied to analyze the chilling-tolerance coefficients(? value)of the indices and evaluate the chilling tolerance of the melon germplasms.The results showed that obvious variation was observed with the ? value of both morphological and physiological indices of the seedlings(CV > 10%)under low-temperature stress,particularly for the ? value of physiological indices.By means of principal component analysis,the primary 17 indices were changed to 7 separate comprehensive indices with a cumulative contribution rate of 84.64%.On the base,the comprehensive evaluation value(D value)for each melon germplasm were obtained by calculation of their membership function values and then weight the values with the contribution rate of the 7 principal components.According to the D values,all the melon germplasms were clustered into3 groups,chilling-tolerance germplasm(one accession),the medium chilling-tolerance germplasms(11 accessions),and the chilling-sensitive germplasms(7 accessions).Finally,the stepwise regression analysis was used to establish a mathematic model (forecast accuracy>93.0%)for evaluation of the chilling tolerance of melon seedlings.D=0.048+0.048POD-0.119SOD+0.097PRO+0.042CRI+0.084RDW+0.206 OFW.(2)The variation of Chilling-resistance about core collection of melon:The cold-tolerance index H 'of the melon seedlings was 1.93,which indicated that the melon seedling was rich in diversity and might have excellent genes.The coefficient of variation of cold tolerance index was 16.32% and the maximum was 32.02%,which indicates that the thickness of the thick skin material was different under different temperature.RI was negatively correlated with CII.RI was significantly correlated with autumn CII,and the correlation coefficient was 0.260.The cold resistance of melon seedlings under low temperature stress was higher,and the cold tolerance index H'was 1.41,especially the autumn recovery index coefficient of variation as high as 60.22%.The change of CII and RI in spring was basically the same,and the consistency of autumn was not obvious.In Spring CV and H' of CII were greater than the corresponding values in autumn,In spring CV and H' of RI were less than the corresponding values in autumn.The correlation coefficient between CII and autumn RI was 0.06 in spring,and the correlation coefficient between RI and RI in autumn was 0.146,and the correlation was not high.It was found that CII had a significant negative correlation with RI,which indicated that the more severe the damage of seedlings was more difficult to return to normal growth.(3)Genetic diversity anaiysis of melon by SSR:Among the 191 SSR,973 bands were amplified in 192 materials,and the number of alleles(Na)detected by different SSR markers was 2 to 13,and the average number of effective genes was 5.094.The polymorphism information content(PIC)ranged from 0.148 to 0.888 with an average of 0.531.A total of 954 bands were amplified in 124 thick-skined materials with an allele number(Na)of 2 to 13 and an average number of effective genes of 4.995.PIC amplitude of 0.119 ~ 0.882,the average value of 0.504.A total of 701 bands were amplified from 68 thin-skinedpopulation,the number of alleles(Na)was 2 to 11,and the average number of effective genes was 3.709.PIC amplitude 0.03 ~ 0.857,the average value of 0.298.The above data indicate that the 191 SSR locus selected in this study have obvious variation in the melon germplasm,and the genetic diversity of the tested materials was rich,and the genetic diversity of the thick-skin melon was higher.(4)Analysis of Cluster and population structure: 192 germplasms were divided into two groups(???)by using UPMGA clustering method.A total of 71 materials in group ? consisted of 66 thin-skinned germplasms,9 thick-skinned germplasm,Group ? contained 115 thick-skinned germplasm and 2 thin-skinned germplasms.Structure analysis could divide 192 melon materials into two groups(P1,P2).P1 contains 121 germplasms,117 thick-skined germplasmsand 4 thin-skined germplasms,was a typical thick-skined subpopulation;P2 contains 64 thin-skined and7 thick-skined germplasmsof 71 germplasms,was a typical thick-skined subpopulation.There were 115 germplasms overlap cross the P1 and ? of NTSYS2.1 clustering results.There were 71 germplasms overlap cross P2 and?subspecies,They showed a high degree of consistency between the two analysis results.(5)Linkage disequilibrium: There were 1278 pairs of locus in the thick-skined group with a high LD state(D'?0.5),which included 951 pairs(P <0.01),accounting for 5.24% of the total number of combinations;8630 pairs of locus were in a high LD state(D'?0.5),and 3669 pairs were matched with the statistical probability P <0.01,accounting for 20.65% of the total number of combinations in thin-skined group.The LD of Thin-skined melon population was higher deg,more suitable for correlation analysis.R2 of the SSR locus on the same linkage group was selected as the index,and it was found that LD was prevalent in both populations and attenuated as the distance between sites increased.The average distance of this study was about 1.2 c M,which can meet the requirements of association analysis.(6)Association analysis:The correlation between the markers and the cold tolerance index in the thin-skinned population and the thick-skinned population was detected by the general linear model(GLM)with the Q value of the thick-skinned population and the thin-skinned population as the covariates.A total of 107 locus were significantly associated with cold tolerance(P<0.05),of which 28 were significantly(P<0.01).52 related locus were found in the thick-skined germplasms.CMCT505 and SSR018207 were repeated under two environmental conditions,and the phenotypic explanatory rates were above 0.3.Among the thin-skined germplasms,55 correlations locus were found.HNM40 was repeated in two environmental conditions and the phenotypic interpretation rate was above 0.1.CMCT505,SSR015399,SSR015829,CMCTTN175,CMTAN142,ECM182 and ECM88 were detected in two different populations,and CMCT505 was repeated in different environments of different populations.SSR015399,SSR018207,SSR010203,SSR010993,SSR032536,DE1329 and SSR031575 have different characteristics in different environments and different groups.SST015399,SSR015399,SSR018207,SSR010993,SSR032536,DE1329 and SSR031575 have different characteristics in different environment and different groups.And as to the repeatability among them,CMCT505 was not only reproducible in different groups of different environment and the explanation rate was very high.Many researchers also found that CMCT505 locus was related to resistance and disease resistance.This loci was probably the main QTL associated with cold-tolerance of melon seedlings.(7)Exploration of elite alleles and tipycal material: In this study,the allelic variation of CMCT505-203 and SSR018207-258 were observed in spring and autumn,and the corresponding vectors were 702,837 and 686,895 of thick-skined melon.The allele variability of HNM40-200 and typical materials were 291,384 and 1066 in spring and autumn of thin-skined melon.It was noteworthy that the explanation rate of CMCT505 was above 0.25,and AEE was significantly reduced in the CII.The AEE efficiency was higher in the RI,and the allelic mutation CMCT505-201 was obviously beneficial to improve the cold tolerance of seedlings.The allele can be used as an important reference for screening cold-tolerance germplasm. |
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