QTL Mapping,Transcriptome And Cytological Analysis Of Fruit Cracking In Watermelon

Watermelon,one of the important horticulture crops,plays an important role in agricultural production in China.According to the statistics of the national industry technical system,China’s watermelon industry ranks the highest in the world,the area and output accounting for more than half of the world’s.The country’s watermelon planting area is nearly 2 million hectares.Fruit cracking occurs during the period of watermelon growth and development,causes negative effect in fruit marketability,and the income of planter has decreased significantly.Although most of the main varieties in production have better quality,they all have higher rate of fruit cracking.It is very necessary and urgent to carry out in-depth systematic research on the mechanism of fruit cracking.There are many factors that influence fruit cracking,including genetic,environment,morphology and physiological aspects,while molecular and cytological mechanism of watermelon fruit cracking is weak.This study constructs a high-density molecular marker genetic map,carried out cytological oberservation,QTL and transcriptome analysis by recombinant inbred lines(RIL)and their derivate near isogenic lines(NIL).It is based for exploring key genes related to watermelon fruit cracking,and improving the fruit crack resistance of watermelon germplasms.The main research results are as follows:1.Comparative analysis on pericarp cell structure between anti-crack and easy-crack watermelonThere were significant differences in cell size,arrangement and tightness of pericarp cells during different developmental stages of resistant and easy crack watermelons by microscopic and transmission electron microscopy(TEM).With the progress of fruit development,cracks or fractures appeared in the cell wall of easily-crack watermelon,and its cell wall was dissolved,the middle rubber layer was separated between the robes,the gap between the walls was generated,the organelle was vacuolated,and the chloroplast granule layer was gradually degraded and aging in advance.Therefore,the breakage of the cell wall leads to the overflow of the middle rubber layer may be one of the reasons for the fruit cracking in the fruit expansion period.The early degradation of cell structure,pectin and chloroplast in the process of fruit ripening,and the imbalance of energy metabolism may be a major reason for the fruit cracking in fruit ripping period.2.Construction of high-density molecular marker genetic maps by high-generation recombinant inbred lines(RIL)We construct a high-density genetic linkage map with RIL(F8)generation population by whole genome resequencing and retriction site-associated DNA sequencing(RAD-seq).The 13664 SNP markers were ultimately mapped to 11 linkage groups with an average of 1242 markers per linkage group.The genetic map finally obtained a total map distance of 2319.62 cM,an average genetic distance of 0.17 cM,and a maximum genetic distance of 53.01 cM.The high-density molecular marker map lays the foundation for the QTL discovery of the agronomic traits of watermelon.3.QTL mapping analysis of fruit cracking related traits of watermelonThrough the correlation analysis of the main agronomic traits of RIL population under different environmental conditions for 2 years,the length of cracked fruit was significantly correlated with the cut cracking,the thickness of pericarp and the degree of wax powder.The field trials were conducted for two seasons,and nine and eight QTLs associated with fruit cracking were mapped,respectively.Among them,the interpretation rate of QTL on the phenotypic variance of different environments was 0.06%-35.78%.These traits were located in adjacent regions,and the QTL intervals were overlapped,which suggests correlation between these traits.4.Transcriptome sequencing technology to analyze differentially expressed genes(DEG)between anti-crack and easy-crack watermelon inbred linesUsing parental inbred lines W11(anti-crack inbred lines)and W13(easy-crack inbred lines)and their derived near isogenic line W96(anti-crack inbred lines)and W85(easy-crack inbred lines)as materials,we analyzed the different expressed genes(DEG)related to anti-crack and easy-crack inbred lines by RNA-seq technology.290 differentially expressed genes(DEG)were detected in "W-1311" VS "W-13",and-165 differentially expressed genes(DEG)were detected in "W-96" VS "W-85".There were 56 differentially expressed genes between the four samples.The expression patterns of 14 differentially expressed genes(DEG)related to fruit cracking were obtained by GO,KEGG,qRT-PCR analysis.